Porco.....
121.5 will probably require a pan call - giving the following details:
Callsign, Aircraft Type, Departure airfield, Destination Airfield, Altitude, Nature of problem (Lost! (or temporarily unsure of your position!!)), and any useful line features etc.
The call will be answered by the Distress and Diversion (D&D) cell at the London Air Traffic Control Centre - who are Royal Air Force Controllers. They will use a triangulation system to pinpoint your spot - so any visual clues (roads, railways etc) would help).
Hopefully, a Military controller will join in here and give you the perfect answer!
Good luck - but never be afraid to use the service!