Thermostatic radiator valves (TRV's) are self regulating temperature control valves which can sense the temperature in a room and adjust the temperature of the radiator to the chosen heat setting. This allows you to control the temperature of each room easily. Since they shut off when the desired temperature is reached they can reduce your energy usage and help you save money!
This is the best valve to buy for installing a towel rail as dual fuel as it allows the flow/return to be located on the same side of the radiator - leaving the opposing side free for the element (so no T-Piece is required as with a standard dual fuel set-up with a pair of valves).