Hey,
one floor with BA even if you are type rated on the aircraft they will send you where they need you! Your friend my have 500hrs 757 and go onto the 737 for example!
A friend of mine had a 757 rating with a few hundred hours on type and he went onto the 777!
Plus the 757/767 is one of the most sought after fleet or so i have heard due to the mix of route! ie some short haul as well as long haul with the 767 out of Manchester to New york!
Best of luck