I can't speak for First Choice now, but I used to work at SFB
back in 2000-2001 and Air 2000 commonly used their 757 on such routes this time of year on certain days....loads were lighter so no need to have the capacity. This is low season right now....better to utilize the higher capacity on a busier winter route.
I will never forget how miserable everyone looked once I finished boarding a full 757 headed back to the UK....people worry about doing transatlantic hops on a CO
757, imagine doing a longer trip....and a fuel stop....with 225 people on board.....ouch!