Don't know about the UK, but in the States, all cars have a coolant overflow tank. (Coolant is considered hazardous waste and is toxic.) The radiator cap lets coolant out when it expands, down a small hose to the holding tank. When the engine cools, it sucks the coolant back into the radiator. In order to work, the radiator cap has two seals. One at the base of the fitting is spring loaded for pressure with an inner relief valve for vacuum. There should be a rubber seal at the top of the fitting to force the coolant down the recovery tank hose.
Check your oil. If it looks foamy, you may have a bad head gasket (coolant in the oil). Likewise, if you start the car and get a big cloud of white smoke, it could mean a bad head gasket is allowing coolant into the combustion chamber. At any rate, get it checked ASAP.