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.