In reply to BusyLizzie:
A few suggestions in the south west:
The first two are spring rather than summer, but might interest you nonetheless and, er, most are not exactly the actual marathon distance.
The Grizzly 18/3/2018
Axe Valley Runners.
3500 ft of coast path, trail, road, bog and shingle beach. 1000+ entrants, but not many 1000s, and a great atmosphere.
Starts and ends in Seaton.
Might already be full?
Exe to Axe 8/4/2018
Sidmouth Running Club
Along the coast path from the Exe to the Axe.
Haven't done it, but looks great. A much lower key version of the Grizzly, with 200+ finishers this year.
Roseland August Trail (Red RAT) 11/8/2018
MudCrew
Entries open on NY day.
Along the coast path of the Roseland peninsula in Cornwall.
In the Red version you run about 20 miles. I did the Black route (32 miles) once and DNFd the second time. There were about 100 entrants each time.
There was also a full/half marathon in late August from Princetown in the centre of Dartmoor to the edge, heading south eastwards. Appropriately called the Great Escape. Run by pure trail running. I might do the half next year if it runs again, being attracted by the elevation profile ,which looked flat, then down!
Freedom Racing ran a marathon in north Dartmoor, from Okehampton this September. Look up Dartmoor Highground Series or something like that. I did the 10 mile Yes Tor event on the same day, and it was very low key with less than 50 entrants. The route took in both classic Dartmoor bleakness and a beautiful river valley.