Not really. 12A is the equivalent of PG-13, but 15 and 18 are completely restrictive. They also allow for a much wider range of potential outcomes. It means that a film like The Matrix wouldn't be held to the same standards as Saw which were 15 and 18 respectively.
15 allows you to use the F word without end, show copious amounts of boobage, as well as cock, and get a reasonable level of gore (Final Destination and 300 came under 15). To get an 18 here you need hardcore drug use, more than 2 uses of the C word, huge amounts of gore and pretty graphic sex scenes.
Distributors here seem pretty happy with it as it means they can get to a younger audience than the US, as The Hangover and Ted managed here, whilst not seeming irresponsible thanks to the restrictive nature of the certificates.