I think the # of hours one will need to put in upon one's return to the UK depends on how thorough the US school was.
My experience is that FAA training is very thorough indeed, both at the PPL and the IR levels. Everybody I know personally (and trust) confirms this with their own experience.
OTOH, all the dissenters that I have come across are anonymous posters on pilot forums, and writers of letters published by certain pilot mags
So, 20hrs does seem generous. Still, there are various levels of experience, with the pilot feeling able to do different things at each level, and after say 20hrs post-PPL one may feel able to go on a 100nm cross-country flight (yes I know this is within PPL privileges but that's besides the point); after 100hrs post-PPL one may feel able to fly to southern France, etc.