A good article from Head for Points explaining the whole slot situation. Mind you, it does end with:
"It also isn’t clear how the ‘use it or lose it’ rule will be applied here. Even if BA starts flying each slot between now and the end of the Winter season in late March, will they hit the 80% utilisation target to avoid it being forfeited?"
Flybe returns 86 weekly Heathrow slots to British Airways (headforpoints.com)