Is this the only PG-13 rated movie that got 18 in the UK?

I think it got an 18 on a technicality, not due to the general level of violence. Up until around 10 years ago, any martial arts involving nunchucks for example got an automatic 18 - plus a few unusual but dangerous fight moves as well, even if they didn't show blood or death.

It's possible The Quest contained one or both. If it were released now it would probably get a 12 or 15 rating.