I'd wait before saying one way or another. It's no secret that all innovative countries use hybrid warfare in one way or another. The days where you send your actual countrymen to a foreign land for doing an operation are long gone. You find someone who's sympathetic to your cause, or has some other problems (money for example), and that's that.

Regardless, I still wouldn't call it terrorism, as the target was seemingly a military base that's involved in an active war... the UK shouldn't involve their airbases in the US's war of aggression if they don't want them to become a possibly-legitimate target.