Just thought I'd clear up a few things.
The '12A' UK version of Terminator 3 is in no way different to the 'R' Rated US version.
The studio (Warner Brothers I think?) was happy with a restrictive rating in the US (their main market/income) so I doubt they deliberately went for a 12A rating in the UK. They probably would be happy with a '15' certificate.
What you have to understand about the BBFC - as apposed to CARA (the American equivalent) - is they are not robots. Although their guidelines are enforced, they're not Gospel.
CARA look at violence as violence, usually not taking into account its subject matter or context. This makes their job extremely easy and usually 'parent pleasing'.
The BBFC prefer to go with a more educated approach which often involves members of the public (parents, teachers, target audience etc) when issuing certificates.
If you look at films such as
Jurassic Park,
Lord of the Rings and more recently
Terminator 3 you'll notice they all achieved lower ratings in the UK. This is mostly (although not completely) due to the context of the violence being Creature vs. Human or Robot vs. Human. Now if you were to substitute the creature/robot with another human, it would be a completely different situation.
This is further confirmed with the recent
Spider-man film which was borderline '15' until the public stepped in (consisting of a test audience of parents, teachers, children) and gave it the 'all clear' so-to-speak, therefore achieving a '12' rating.
If you're still suspicious of Terminator 3's rating (which I must admit was pushing the envelope), check the version you watched in British cinemas against a full listing of it's violent/language content featured in the American version
here.