In general it is not simple for a UK-educated lawyer with non-US nationality to practise law in the States. As mentioned above, this is not just a matter of the education and training but of visas and work clearance. The easiest way to work in the US would be if you worked for a firm in London with a US presence and were seconded to a US office for a fixed period.
I agree with Rochelle, and I would ask the posters who want to practise law in the States why they want to do so - a genuine question. Is it that you want to live in the USA in general, or that you think practising law over there will be more glamorous or interesting than here?
A solicitor / barrister from the UK, to be honest, basically does not fit into the US system. Yes, you could take certain bar examinations, or even be exempt (I think perhaps Washington DC exempts solicitors / barristers with a certain level of experience from having to take their bar exam, but don't quote me!) But then you have to get a job, and any employer is going to query why a British person wants to work in the USA and will have to get this reason through the authorities to get a visa / work permit etc. There is likely to be no good business case to do so unless you are experienced in your field in an area a US firm could find helpful; there is more need in reality for US regulatory / securities experience in London than there is for London experience in the USA. The cynic in me suggests perhaps it's media, films, books, TV shows etc that make people interested in practising law in the US.
I have practised law at a US firm in London, which had regularly opportunities for secondments to the US but these were for business / firm-related reasons (getting to know US colleagues etc) - it wasn't a career option for the associate as such. I was sent over to the US myself for a short time. But, there was no opportunity for a permanent move over to the States as the firm had no business long-term need for English lawyers over there.
This question comes up very frequently on legal advice forums and I do query where English people get the inspiration for practising law in the States from - is it books, films, TV? The system is just not set up to make practising law in the US a feasible career option for 99% of people - what's wrong with practising law over here or those places where an English law qualification is useful?