cant see any reason for not putting thermostatic valves on radiators. Although one radiator has to be left with normal valves (bathroom one normally) this allows water to circulate and heat to escape if there's any problems with the boiler thermostat or controls, and the bathroom one is where most people like to be warm. If going to the cost of replacing the boiler, it will probably benefit you more have the system converted to a two pipe sytem.
You can put TRVs on all radiators as long as the radiators are branched off the main pipe with 'tails'. Even if all TRVs are closed in this instance, there is still a flow circuit to/from the boiler. The vent pipe is there to cater for instances where the thermostat fails.
I would agree that it's best to convert the system to a 2 pipe system if money allows.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!