you don't need to go back to school if you get your JD, pass the bar, and have at least 2 yrs of experience. you just take the easy test called the QLTT, which makes it easier to requalify in England than in any other state in the US, since the QLTT is easier than any state bar.
however, the QLTT is uesless. why? because US firms in London (and UK firms) will only hire corporate finance lawyers with at least 1 yr experience in big New York firms, or sometimes, Ivy League grads. they won't even require you to pass the QLTT and will still pay you double what English lawyers in the same firm, with the same yrs of experience, and doing the same hours, will earn ie, 100K instead of 50K (pounds that is).
everyone else would need to take the QLTT, but there would no jobs even with that, and you're not allowed to work solo.