FM stations are fairly local (line of sight stuff, mostly), and the lottle short range things manitas describes are VERY short range. Does it go away if you move the vehicle? Can you still find it? Where are you in the UK?

It sounds like an engineering signal being broadcast to test the transmitters before a new station goes officially on air. The absence of idiots jabbering is usually a dead giveaway. Are any new stations planned within say 30 miles of where you were?