first and foremost parents should be responsible about what games (and films) they let their children play. Ultimately it's their decision and all the blame should rest on them if they bring their kids up to be ASBO wielding cop killers.
The PEGI ratings are a lot more practical as far as helping parents decide what to buy for their kids as they actually show the type of content, not just what age the censors think should be allowed to view it.
BBFC is a censorship board for non interactive content, so even if they hire lots of new censors with all the money they force out of what's left of the UK games industry they'll still not have the experience of rating games that PEGI does.
Forcing BBFC ratings on UK games doesn't hurt the big potentially 18 rated games as much as it could the children friendly casual games too. Casual games have to work to tight margins, so slapping extra charges on them for releasing in the UK (as well as UK VAT, overvalued Sterling and higher cost of production here) means publishers may have to consider whether or not it's worth releasing their game in the UK.
Take away the last little incentive of seeing the games you make on shelves in your own country could be the last straw for dev studios eyeing the huge tax breaks they could get for moving to the US, Canada, France or pretty much anywhere except the UK.