In my previous house I thought it was because my TRVs were cheap and nasty that they didn't close completely. This made it impossible to remove a radiator for an extended period of time, as the TRV was dripping at the rate of a maybe drop per second at ~1 psi pressure even when closed as tightly as possible.
In my current house I have brand new, well-known UK name, good quality, expensive, reversible flow TRVs. I decided to move the misplaced radiator in the dining area, and thought I could disconnect it and hang it in its new place before starting on rerouting the underfloor pipework.
Fat chance. Not even violence will stop the TRV from dripping when the radiator is disconnected. Are there some stupid technical reasons for why TRVs should not be able to close completely???