As things currently stand, I believe Wirbelsturm is partly correct. Certainly in as much as crew must report 'acclimatised' for their trip. This alone would certainly cause problems for some of our commuters who have to travel significant distances to LHR. There are of course also tax implications in spending nights in the UK! However some of these same (LH) commuters hold influential positions within BALPA, so I guess watch this space.
As far as Back to Backs are concerned, I believe there is nothing in EASA to explicitly forbid them, however they must be constructed by the company, who must provide 'approved' accommodation between the flights. No such agreement currently exists. Nor can I particularly imagine BA being terribly interested in the subject unless pilots 'buy' such a process?
However as I said, some of our (LH) long distance commuters occupy influential positions within BALPA, so I guess watch this space.
Commuting certainly won't be impossible, but it is likely to get a little more difficult? Nobody yet knows by how much? If this is a deal breaker for you, then you might be better looking elsewhere?