I am somewhat surprised to detect a reluctance to file a flight plan. Notwithstanding that the destination may not be on the AFTN, I believe that the Welsh Mountains are considered difficult for Search & Rescue.
If you file a flight plan and you are overdue the authorities are obliged to take appropriate action. OK it takes a little bit more organising but I remain to be convinced that filing a plan is not an appropriate course of action for this type of flight.
The UK AIP contains advice/information where the aerodrome is not on the AFTN.