I am amazed that someone asks this @Hugglespuffed
The College police guidance people linked to is for the police who may stop people and ask why they are driving etc.
It doesn't mean you can or ought to aim for doing exercise a long way from home or drive to a beauty spot or National Park (and most UK mountains are in National Parks.)
Unnecessary journeys are still not allowed.
The guidance is that you can exercise but that it is close to your home.
You can walk the streets or go to a nearby park.
Presumably you live in Scotland , Wales or the Lakes if you want to climb a mountain.
As far as driving goes, the guidance is that no one should do unnecessary driving.
The way I'd think driving was remotely reasonable is if, for example, an elderly person wanted to walk for an hour on the flat, but had to walk up a very high hill or bank from their house to get out to walk and they weren't fit enough or able to do that.