Plenty of genuine F passengers book last minute, change late in the day etc.

Anecdotally, BA is rumoured to have lightened up on upgrades into F although, if you think about it, there isn't that much true F competition for BA JKF/LHR. It's probably one of their top routes for F sales - so you'll probably be rubbing shoulders with fellow pax who've paid out/or had someone pay out big time on their behalf. Should make you feel a bit better - assuming you're cashing in miles.

If you're on the 112, it was 14/14 in the nose the last time I took it.