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Your understanding does not go as far as you thought...
You have to be able to disable the radiator in the conservatory, not run it when the rest of the system is off. Think about it - Part L is all about energy conservation and not chucking out heat to the night sky in the middle of winter when the conservatory is not in use.
A TRV to control the conservatory radiators is accepted as being fully compliant with the requirements of Part L (and exemption to building control).
If one wishes to heat a conservatory in the absence of heat demand in the rest of the house (e.g. cooler summer nights for example) then using the main boiler for a single zone of only one radiator makes very poor economic sense - you are far better off using supplementary electric heating. A basic standalone convector will more than suffice in such circumstances.
Mathew