The small bore aspect of this isn't a problem. You can get a set of brass reducing bushes that will enable you to fit the new valve direct to your existing pipe. Just make sure that you ask for the correct size in the plumbing shop. You will almost certainly need an 8mm to 15mm set.
You shouldn't really be turning a radiator on and off every day - just get a TRV and let that do the job of regulating the flow rate.
It came with the TRV. As I have Boiler maintainance cover that includes pipes and valves, it is they who do the repairs. I have asked them to just fit a normal tap valve, but they say they can only replace the existing TRV, even though a tap would be cheaper.
FE, it already has a reducer. I turn it off each night as the conservatory is closed off each evening, and it gets very cold in the night.