It's probably a mixture of several things.

The percentage of students has doubled over the last 20 years but many of them aren't doing traditionally 'academic' subjects. Which might push grades up generally. Plus there is more pressure to get a good grade - since everyone has a degree.

I'd also imagine that some of the courses are just getting more competitive. Have universities also doubled in size? If not, then you'd expect the kids going to Russell group unis to be a bit harder working, while some smarter kids will now be going to less prestigious schools.

And it seems intuitive that kids today would be doing a bit better at studying than kids in the 90s. They're more likely to be raised by at least one parent with a university degree, their access to learning resources has exploded in the last two decades, and even kids at crappy comprehensives are now trained to go to university.

Then there probably is the more cynical grade inflation happening. Attitudes about grades have changed. It used to be that getting a 2:2 wasn't a failure. Now most kids would think, "you got an average of 54 over three years in a Mathematics degree? Did you even attend class?"