Translation:By now he lives in Great Britain again.
Leben can be used to express that he "resides" somewhere, too, but also adds substantially more meaning than just wohnen. So leben here means that it's really home for that person, not just where he resides. Mittlerweile would signal that the person was originally from the UK, moved away, and now is back to his home turf.
Yes, 4 years since I asked the question above I think this is a really bad sentence to introduce mittlerweile with, since the meaning conveyed here makes reference to a previous statement which is missing, e.g. "since we last talked about him / mentioned him, he's back in the UK".