A TRV body is connected to the water circuit. Removing it opens the pipe on the system side as well as the radiator side. Closing that same valve does not protect the opening left when the body is taken away.
This is a competent plumbing job with an identified method, not the next required exercise after changing a removable head. You can understand controls and balancing without doing it. Work on the gas boiler itself, its final water connections, gas fittings or flue belongs with the appropriately registered engineer.