Well, most people are not as obsessed with their seat assignment as those on Flyertalk. And even by Flyertalk standards, crackityjones is pretty obsessed.

Most people are not willing to tie up £5k or whatever the J-fare is, for a somewhat unspecified period of time, just to get a better seat. In fact, I think hardly anyone would be. If it became a regular practice I'm sure BA would crack down on it, so to speak - they'd find a way. And if the OP tried it more than once, they might well notice: smoke detectors don't just work with tobacco.