Update 2: I've written a practical guide for emigration. It contains some very concrete suggestions, including if you have few specialist skills which might get you abroad.
I keep getting asked to how move to the UK and I figured it was about time that I present useful information for people. However, I am not a lawyer, nor am I an immigration specialist, so please double-check everything I write. I make no guarantees.
Also, much of the following information applies to any country you want to emigrate to, but there are some UK specific tidbits, hence the title of this post.
First and foremost, you need to be positive you want to do this. Most people brush this off, but the sad reality is, homesickness is a very tough thing to overcome. More than one person on the FBI's witness protection program has been murdered because they took a quick visit home. Think about that -- homesickness is so powerful that people risk death for it. Frankly, I've struggled with it, but I'm lucky to have family and friends over here in the UK, not to mention a stable job.
When I say "positive", I mean that you need to articulate a clear reason why you want to emigrate because that's always one of the first questions potential employers ask. If you say "I hate the political situation in my country", they're going to know darn well that you may very well hate the political situation in their country. Heck, they might admire the US and not hire you just because of your answer. Also, "adventure", while possibly true, is not a good answer as it's extremely vague. If you don't get enough adventure, are you just going to go home? Besides, when you say "adventure", they'll hear "dreamer" and that might not be what you want them to hear. I heavily stressed the fact that I had family in the UK and wanted to be closer to them. Whatever reasons you come up with, make them sound very positive.
Second, do not even think about doing this illegally. If you are caught, not only will you be deported, you probably won't be allowed back and immigration forms for other countries regularly ask if you've been deported from any country. Lying about this pretty much means that you're stuck in your home country.
Third, be very patient. This process takes time and you have to be really dedicated to pursuing it. That might mean building up your resume, waiting a long time for responses from companies you contact, or assessing the laws of other countries you're willing to move to. Myself, I started in January and didn't get approved until April and didn't move until June. That was probably quicker than many others, but the truth is, I've wanted to do this for years. I just knew I had to build my resume to the point where I could get away with this.
Fourth, make sure you've actually been to the country you want to move to , if only for a vacation. Also, do as much research on it as possible. You want to make sure that you can actually function there and employers want to know that, too. Heck, in an interview, if you get one, make comments about their country which are not only both positive, but also show that you've done your homework.
Fifth, relax! Your potential employers are humans (probably). They're just as likely to be new to this situation as you are. It's easy to talk to them and if you have sound reasons for going over and are sincere, they'll probably take you at your word. If they have plenty of experience hiring foreigners, it's easier for them to bring you in, but they also have a better of what questions to ask, so be prepared.
Finally, be picky. If you do get an opportunity but it seems questionable, ask yourself if you would accept it in your home country. If the answer is "no", you should probably move on. When I moved to Amsterdam, I fell in with a company that was failing and had a terribly production environment. I would have turned them down if they were in the US. As it stands, I had three potential employers in the UK and I deliberately chose the employer who was offering the most stable work environment as stability was my biggest criteria for accepting a job.
Now for the strategies:
The easiest way to move to the UK is to fall in love with a British citizen, marry, and move over there. Of course, that's "easiest" in terms of qualifications that you need. It's still up to you to find that Brit and make him/her love you and agree to marry you.
Another possible way to get into the UK is if you qualify under their Highly Skilled Migrant Worker (HSMP) program. This program allows folks who are extremely skilled in their field to live and work in the UK without a work permit! This is great, except for one little problem. You need 65 points or more and this unofficial points calculator will probably show that you won't qualify. It helps if you're young, make more than $70,000 a year, have a master's degree or better, have worked 5 to 10 years in your field and have national or international recognition for your work. Having a spouse with a college degree also helps. If you meet all of those criteria, you still might not qualify and the national or international recognition is hard to achieve. I know of at least one individual who I thought would be a shoe-in yet was turned down anyway.
2007-09-15 Update: The HSMP scheme has changed! This change affects people already on the HSMP and has screwed over many people who have emigrated here. It's now the Highly Paid Migrant Program. Of course, they really don't call it that, but they no longer give a damn if you are highly skilled and recognized. The (mostly) only care if you have a college degree and a fat paycheck. There are people who have given up their entire lives to move to the UK and are now facing deportation because the government has changed the rules on them. It's a bit of a political hot potato right now. It could happen again -- particularly with anti-immigration sentiment growing here.
The traditional way, though, is to apply for a work permit. For the UK (and for most countries, to be honest), you need to start submitting your resume to employers. If an employer offers you a job, they can apply for a work permit for you. You cannot apply for one directly.
To qualify for a work permit, the employer has to have a vacancy for a high-demand job, they have to prove they're a legitimate employer (in other words, not a company created for the sole purpose of bringing your sorry ass to the UK), they really need that position filled and that they could not find a single person in the entire European Union (well, EEA, actually, but that's a long story) who could both fill that position and was willing to do so. They even have to prove that they appropriately advertised for that position. If they fulfill all of those criteria, then you have to be able to prove that you're qualified to do that job!
However, if a company is willing to offer to sponsor you, you're probably good to go. The trick is to get a company to be willing to sponsor you. Here are a few suggestions.
First, you need to be somewhat recognized as an "authority" in an area generally related to what you do. There are several ways to do this. For myself, I simply started hanging out on Web sites and started answering programmer's Perl questions. Eventually I was confident enough in my skills that I managed to get a couple of articles published and somehow wound up with my name on the cover of a book. That coupled with a strong resume made it somewhat easier.
One thing which can't be emphasized enough is getting your name out there. If a potential employer asks one of their employees if he/she has heard of you, a resounding "yes" is a strong point in your favor, unless it's followed up by "and that person's a jerk", so make sure you are very considerate online. I know of a few well-known Perl programmers I would not hire simply because they're obnoxious online, despite the fact that I'm told one of them is a real sweetheart in person. Heck, use Google to search for yourself online. You might be awfully surprised at what you find (apparently I'm a Sewage Authority Executive Director. If you've seen some of my code, maybe you'd agree.)
What this boils down to is an employer picking up your resume and seeing something on it that the other resumes don't have. "Published articles" is great (and easy -- magazines are often desperate for content). "Patents" are hard, but also great. Participation in well-known open-source projects is probably better than being the author of unknown open-source projects. Listing your "speaking gigs" is also a plus. In other words, you need something that makes you stand out from the crowd.
If an employer does sound interested in hiring you, make it easy for them. If they don't know how to apply for a work permit, give them direction. If you're willing to sell all of your stuff so they don't have to ship it, tell them that. If you own a home, how are you going to deal with that? In sales, these are known as "objections" and you must anticipate and overcome them all.
There's a lot of legal documentation you will need to fill out. Make sure you visit the British government Web sites and download everything you think is pertinent to your situation. Then make sure you can get all of the supporting paperwork beforehand (letters from former employers, passport (duh), higher education degrees, and so on). This will save you a lot of time and trouble.
I should mention that these are not the only ways to get to the UK, but they're the most common. There are programs for artists, writers (I'm lookin' at you, pdx42), professional athletes, domestic help (rich people can bring their butlers), retirees, and so on. Do some digging and figure out how to make it happen.