I repost this from elsewhere:
Simply put:
There are the following routes:
1. Do your degree then do the LSAT and go to a US law school for 3 years, and sit the JD. - This is expensive and additionally VERY competitive. Getting into a tier 1 school is hard enough, but doing well at one - (and really you should aim for top 5 [Yale, harvard, Stanford, Columbia, NYU]) - is even harder.
2. Do your degree here, then do a GDL CPE, then do an LLM in the US (you will actually have to check which LLMs you will be able to sit, not having done an LLB). Again - more expense, and further - many Biglaw dont really care about LLMs, aside from perhaps a tax specialisation (if so go to NYU).
3. Join a US firm in the UK, qualify in NY as well, and after a few years, ask for a transfer to NY.
4. Just sit the NY bar if you are able to after your conversion (Im not sure if you can though - but if you had an LLB you certainly could).
Now... the fact of the matter is that getting the degree or being ABLE to practice in the US is neither here nor there. What DOES matter is being able to find a good job afterwards. Without a JD and/or STELLAR grades over here for a TOP uni, and a GREAT LLM in the US (again see top 5), you will find it very hard indeed to get the best jobs, or for that matter, any of the mediocre ones.
IMO - and this is a route I have taken - the best thing to do is get very very very good grades over here, and join a US firm in the UK to begin with (choice 3). This is more stable, cheaper (by far) and far less risky.
Hope that is of some help.