Flying in the UK is more hassle and expense than anything else. Doesn't matter if its a rental C172 or a trying to fly for the airlines here.
Bottom line is, your FAA license on its own with current medical will suffice. No conversion or validation procedure is necessary. But some flight schools try to make money out of you by insisting on 5-10 hours check out, whilst others simply shy away from FAA pilots altogether.
The Radio work is very different (super anal)
The VFR map is colourful but confusing, the air traffic control procedures is different.
The weather is poor and unpredictable most of the time (rain, low cloud, grey skies, wind, FOG)
The cost is exorbitant!
But realistically you should aim to do atleast 3-5 hours dual with an instructor before you go solo, plus pay for landing fees at every airport you go to. Its the same airplanes as the USA, but just a totally different flying culture here.
Thats why so many Brits/Europeans learn to fly in the USA first, then convert to the EASA. Its cheaper. Like for every one dual lesson you get in Euroland, you can get 3-4 in the USA with instructor! And NO LANDING FEES (at most GA airports anyway)!
As a side note; As far as a career in England/Europe goes, well after spending the equivalent of over $100,000 USD to finish flight school with your new EASA Frozen ATPL (CPL with 200 hours TT and ATP's written), just be prepared to get in a very long line and wait many years before any airline will hire you. Their are just too many pilots in the pool here, and not enough jobs. And the legacy careers only go for the ATPL, 2000+ hours and time on type guys, so the reality is most who have just finished their commercial will never get to exercise it!
Its a bad scene here!