Probably Orthopaedics these days. Basic salary from the NHS (full time, which I think is 7 sessions a week these days) of approx £70k, then you add onto that with your private practice. The majority will be earning in the £150k area, if you want to work and not do much else you can go over the £200k mark...
Cardiothoracics used to be a good bet, but with all the stenting etc they're doing these days, I'm not sure how long that's going to last.
You want something that has a good private practice to earn money over here - so, something common, fairly quick operation, preferably one with quite a long wait on the NHS, doesn't usually require ICU facilities.
Orthopods have it made - just think of all the knee and hip replacements!