Adder?

Yes, looks like a melanistic Adder - you can see the faint markings of the zig-zag pattern along its back (female I think, looking at the tail). They aren't particularly common, but melanism is a genetic condition so you often get a reasonable local population in an area as the genes get passed on, yet a few miles away they're never seen. We see them at Minsmere for example & I would guesstimate they form somewhere around 5-10% of the local population based on personal sightings. The melanism does allow them to warm up faster (and they're born 10-15% bigger) which are obvious pros, but the con is that they're less camouflaged making them more prone to predation. The two seem to balance out, so they never become the dominant colouring

Rowan Atkinson?