I read somewhere else a quote that referred to the first 7 A350s being based at LHR & MAN, but the quote in that Travel Weekly article only mentions LHR.
Weiss said: “The first seven will be based at Heathrow, then five at Gatwick perhaps by 2021.”
So unless some of the a/c are rotated through MAN, it would seem we may have to rely on the A330s when the B747s are phased out.