The 12A became the new 15 years ago, with the 15 becoming the new 18, the tacit admission from the BBFC being that very few if any commercial films with wide box office appeal will have content necessary to have an 18 slapped on them.
I would favour ditching the 12A. We've all seen what it's become, and how the rating and BBFC are being manipulated and misused by the likes of Fox. I would also ban the BBFC from being able to offer pre-classification advice to distributors. Actually, I'd ban the BBFC from cutting films at all. They should classify and nothing else. Every film is what it is. It should be classified in its whole, and made available to the appropriate audience. Film companies and distributors are well aware of what will and won't pass specific age categories.
Banning pre-classification advice would just make things worse. Studios would cut blindly rather than cutting the minimum possible, which is what the BBFC's advice allows them to do. Whilst many agree cutting for a 12 is a bad idea, how much worse would it be if studios just pre-editted the film to suit, probably going on the safe side just to be sure.
Also, what about films which breech the law or contain elements which breach the law (eg unsimulated cockfighting) do they just get banned from distribution in the UK? Banning the BBFC from cutting elements of films will only make things worse, as they will be forced to ban films they wouldn't have had to before...