BOP in the BTCC keeps the privateers and random brands in, not the manufacturers. Half the cars have that generic Swindon engine IIRC anyway. The factory supported teams would've been in regardless, it just wouldn't have 20+ cars on the grid.
It is not BOP though in BTCC, it is actually give more boost to certain cars at certain points in the season. It is over balance of performance for some cars over others...
Take 2 good drivers.... give them the same car..... give one car 25bhp more..... who is 'fastest' driver?