I considered that term, but the ones I like the most are made from heavyweight cotton twill, similar in weight to denim but with a different weave and feel. Whereas I think combat pants are generally some sort of polycotton or cordura ripstop fabric.
So it's "service" as in service tech or mechanic, not military service.
Another UK commenter - over here that cotton twill is known as moleskin and a lot of the army surplus is in that fabric which I agree is much nicer than the ripstop stuff
In the UK we might call them "combats" or "combat trousers", and the best ones tend to come from military surplus stores.