You never can tell with BA. When I joined the airline I was asked which fleet I would prefer. The 757 was one of the fleets offered, so it was one of my first choices. At this point the 737 at LHR wasn't even a twinkle in Bob's eye. A while later the letter arrived saying congratulations you'll be going to LHR ... on the 737.Everyone I spoke to said I was going mad (been happening for a while now) and that there was no 73 fleet at LHR.
Anyway, to cut a long story short, it seems they had announced the reduction of the 757 fleet and advised pilots to bid for a fleet move unless they didn't mind being frozen on the fleet till it was closed. Weeks later, after the more senior flight crew jumped to other fleets, they announced they would be keeping the 757s for a bit longer after all. Most of the next 50 CEPs to join BA went to the 757. I bump into them from time to time at the Compass centre and they say 'Hi where are you going?' I say 'Aberdeen ... again' and they say 'Aw never mind. I'm off to Nassau.'
At the moment you can expect A319/320 (LHR and BHX), or B737 (LHR, LGW and possibly MAN). Just remember the plan may be different tommorrow.