The South West Coast Path is the longest walking path in the UK. It runs 630 miles along the southern and western coasts of the UK, through Dorset, Devon, Cornwall and Somerset. The SWCP has seen several FKT trips in recent years. Here's a summary from Damian Hall's post on the inov-8 blog (with links added):

The Selvaggio Blu is a multi-day backpacking expedition along a remote limestone coast overlooking the sea with difficult route finding, abseiling and some climbing. 
The main challenges are the route finding, although use of GPS makes this easier but certainly not foolproof, and the lack of water on the limestone terrain. This is a route for mountaineers and people with alpine mountaineering experience.