Your correct 747-436, BMED should have had the 757 and where offered 757s by BA but they declined.
The 757 slots in nicely to the network. Lets not forget its NOT about vast passenger numbers, but lots of business class paying pax, CARGO and going direct. The A321 couldn't offer us Cargo or direct services.
When it was all to late BMED approached BA to get widebodies (A340s) but this was rejected by BA as was pretty much any new route BMED wanted to do.
Until the sale (if its for sale and we all know its really for sale) of bmi is complete leasing out the routes to a 757 operator will generate more ££ in the bank.