I saw an adder when I was off-roading (on a path) on my bike just outside Stanhope in County Durham.
It was about a third of the way up Crawleyside bank, and seemed quite content basking on the chalky path.
A quick google suggested there is a known population in the area, and the adders tend to like some elevation, which it had where I found it.
There's some concern about inter-breeding in the population because the ever busier roads prevent the snakes from travelling - and mixing - as they would like to.
I've no idea how far a snake likes to slither on a day out, but it seems adders will travel miles given the opportunity.
It's always struck me as cruel to keep a snake in a glass box.