So asks a law student. While I know various American students who have gone off to practice in London, I actually don't know the answer to this. Comments are open. Insight and links to resources welcome.
UPDATE: Bill Burke-White, a lawyer who is currently a lecturer at the Wilson School at Princeton, e-mails the following exceptionally informative response (some of which is similar, I see, to what Professor Froomkin has recently posted in the comments):
"In response to the question about US trained lawyers working in the UK, there are a couple of ways to do this. I'll try to outline the main paths here. First, both US and UK law firms hire US trained lawyers to work in their London offices. Most of this work tends to be securities and project finance. These lawyers are effectively practicing US law (generally NY law) but doing so based in London. There are no special requirements for doing this other than getting hired for a London office and then passing a US bar exam (normally NY). These positions can be quite lucrative as lawyers make an NY salary plus ex-pat benefits.
"A second option is to actually qualify as an attorney in England. As you may know, in England there are two separate routes to legal practice. Solicitors work directly with clients and do a range of corporate transactions and preparation for litigation. Barristers (the ones who wear the wigs) are then hired by solicitors to provide legal opinions or argue cases in court. Though barristers tend to work as part of a chambers, they are independent rather than salaried or part of a partnership. This is not quite the same as the corporate/litigation distinction in a US law firm as they are fundamentally different career tracks, but that can be a useful reference distinction.
"Assuming you have a US law degree, to qualify as a solicitor in the UK you must pass a US state bar exam and gain two years of common law practice experience. Then you can complete the Qualified Lawyers Transfer Test (far easier than a US bar exam). Many US trained lawyers working in London go ahead and qualify even if they do not plan to practice English law. For more information on this route visit this site. It is worth noting that most English lawyers undertake legal study as an undergraduate degree and therefore begin work at a younger age. They are therefore required to complete a 2 year traineeship at their solicitors firm before becoming fully qualified. Most firms will waive this if you qualify from abroad after 2 years of experience. As debt burdens are lower in England, solicitors firms tend to pay significantly lower salaries for English lawyers than a US firm would for a US qualified lawyer.
"Becoming a barrister is far harder. The typical route for a British lawyer would be to complete their normal legal training followed by a year of Bar School (often at the Inns of Court School of Law) in which one learns the practical elements of court appearances, etc. Then you must get offered a pupilage at a barrister's chambers which are highly competitive. Some proportion of those offered a pupilage will, after a year, be given a tenancy. Once you have a tenancy at a barrister's chambers, you are formally called to bar, have an independent legal practice with rights of audience in court.
"For a foreigner to become a barrister, you have to pass a bar exam in your home jurisdiction and have 'regularly exercised rights of audience in the superior courts of a common law jurisdiction for at least 3 years,' have UK work status, and have reasonable grounds to expect that a barristers chambers will offer you a pupilage. These are fairly difficult standards for a recent US law graduate to have. Often the easiest route to being a barrister is actually to get a job at a law firm in the UK, qualify as a solicitor, get work status, and then complete bar school and then get offered a pupilage with a chambers. There is one shortcut. If you have an academic appointment as a teacher of law in England, you can automatically get called to bar and skip all the above. Useful information can be found at this site."
