Also, you should realise that it isn't the SAME few 737's outside the hangar. They are there for routine maintenance. This morning when I drove past, there was an A319 there too. You're theory is pants.
As a side note, the condition of the BA 737's is a lot better than the EZY 737's. Having operated on both, I'd much rather be on a 15 yr old BA 734 than an 5 yr old EZY 737. Of course both pale in significance to the 319
It's just down to how the airline's look after their a/c. EZY have their planes in the sky a lot longer and no cleaning after each turnaround except that done by the crew. BA have longer turnaround times which will included a proper clean and so stay in a better condition longer