In reply to Derry:
There are also the Classic Quarter which I did (just), from the Lizard to Land's End, about 44 miles, and the Exmoor Ultra (plus shorter distances), both run by EnduranceLife.
Bys Vyken run the Cousin jack, 35 miles from St Ives to Cape Cornwall and back. That's pretty spectacular and tough running for much of it. Shorter distances are available.
I seem to remember too that Climb South west run a coast to coast (N to S of Devon) via Dartmoor.
And, it's not an ultra, but you could do worse than consider the Grizzly, 20 miles from Seaton, run by Axe Valley Runners. 1000+ runners, lots of hills, mud and shingle beaches. Always a great atmosphere.
Many or all of these, of course, are probably either already booked up or not running this year.