Cars driving way too slow

I'm coming across it a lot lately that cars will just kinda decide their own speed limit. Whether it be 50 on an 80, 30 on a 50, 80 on a 100 etc. It's usually either old people or instructor cars. I'm pretty sure this is a road traffic violation no?

I only passed my test 2 years ago and I remember my instructor saying you have to build up to the limit when it's safe to do so. Otherwise you will be faulted for progress. Do instructors not care about this? I know there is the whole "its a limit not a target" but my understanding that means use your best judgement when it comes to conditions, visibility, road surface etc. Not just deciding when it suits you.