I used to see them, mostly along the trails through the bracken woods, warming up in the morning sun.
Occasionally I would spot one among the grass on the edge of the woods, usually they would scurry away pretty quick on the approach of a human but on the grass they were much slower and couldn't get the traction they needed to get away.
Why they're are called grass snakes, I don't know, maybe the clour ?
Often found in garden compost heaps, excellent for incubating their eggs.