As an alternative to flight plan use the "responsible person" system . If you have not called your nominated responsible person within 30 mins of your eta they should initiate overdue action.
I personally wouldnt think of filing to cross the hills but would only cross in clear conditions. Have had some wonderfull trips!
Mona has an a/g at weekends usually no one there but the radio man.I use it for circuit practice dead cheap and every time I have the circuit to myself. Caernavon's ony a hop for fuel.
for a cross country I'd talk London info but not just for a bimble round.
Valley listen out only at weekends for air sea rescue and get dead shirty if you call them probably interupts their telly!