What is this beetle? Found in heather in snowdonia
I believe it's the Violet Ground Beetle - 
Carabus violaceus. This is a close relative to the Carabus coriaceus posted by Jon.

The purple colour is a give away, but as in many insect groups there are often very similar species. As pointed out in the comments to this anwer, this is the case for C. violaceus and C. problematicus (differences)

There are websites  like observado.org where sightings can be added as a species or for instance "unknown beetle", where they are reviewed (depending on the availability of the volunteering experts) or identified.