Hiace,
AFAIK (I'm not a commuter) the EASA FTL's require you to be 'acclimatised' at the point of destination which means spending (I believe, I'm not 100% sure) 2 local nights withing two time zones of your Long Haul base departure airport.
So, if you reside in Europe within +/- 2 time zones of London you're fine for commuting as time at home constitutes acclimatised.
However, a B2B New York etc. or commuting in 24 hours before a trip from the States would not meet the criteria as there would only be 24 hours in the UK between the trips thus making you not acclimatised.
As I say those are only the rules as have been 'discussed' on the flight deck. I'm sure someone may well be able to amend/fill out on the above as necessary.