25 in a 30 is completely reasonable. It's a maximum, not a rule.
Overtaking would need breaking the speed limit, so yes that was stupid of them and dangerous.
Don't feel pressured to drive dangerously because others do.
I once had a fool in a van speed past me in the dark on a blind corner, swerving around me. I was driving at the speed limit, on it exactly, not slower or faster.
When I asked why he said "Em, well, why were you driving so slowly?"
I said, the limit on that road between x and y is 40, prior it's 30 due to x, later it will change to 60, but that's another 14 miles from here.
So, as the speed limit is 40 and it is not raining, I am an experienced driver and I am familiar with the roads, I was driving at 40. I assume the van swerving past me on the blind bend in the dark was doing more than the limit and clearly driving dangerously, did you mean to pull him over? I left room for you to pass as it was clearly dangerous and of course I, although apparently not the guy in the van who you were directly behind, knew you were behind him since x as I use my mirrors. would you like to check anything else?
I don't recommend talking to a police officer like that, but the dangerous speeding swerving in the dark guy was insane and should have been stopped! So I made my point and got an apology rather than arrested.