I stand to be corrected but ......
If you file at FL120 on that route you drop out of controlled airspace just to the west of Bournmouth.
If your above FL100 you sould be able to get a RAS with LJAO Southwest (Swanick Mil) who will help you cross N864 (they don't normally like working things below FL100 because of radar cover).
The TMA controllers should co-ordinate you but I think the frequency is 135.150.
Swanick Mil should then hand you off to St Mawgan 128.725.
If it wasn't August you could get the same service from Plymouth Mil who would do the N864 cross, but it is and they're all on holiday till September.
OR....
If you descend to FL50 after leaving controlled airspace to pass underneath N864 you could probably get services from Bournemouth, Exeter, Plymouth (Procedural only) then St Mawgan.
Plymouth is often by-passed though and you'll be transferred from Exeter to St Mawgan so you will always have a radar service.