RichardDIYPlumber

I have a radiator in an upstairs bedroom. I fitted this myself some years ago and I didn’t attach a thermostat. The room now gets too hot when the central heating comes on. I’ve checked the fitting that connects to the radiator and I can’t fit the thermostat I have without draining down the central heating which I’m trying to avoid. I’ve uploaded some pictures. Does anyone know of anything that would be suitable to attach to this kind of fitting that could act as a thermostat or at least restrict the hot water going into the radiator so it doesn’t get as hot?