Roughly four and a half hours if you take the route recommended here:
http:/www.theaa.com/route-planner/index.jsp
Type London in the from box and Woolacombe in the to box.
Hot Tips for Woolacombe:
- The cobbled hill is very steep.
- The beach is covered in pebbles, practically no sand.
- There used to be a pretty waterfall on the beach, I haven't been to Woolacoombe for over seven years, but it might still be there.