With your existing setup, all you can really do with a room stat is switch
the pump on and off - so, whenever heating is selected by the main
programmer, the boiler will cycle on its own stat - and the DHW will
continue to get hot(ter) by gravity circulation.

That isn't quite true. On an old boiler setup like this, you would set the
programmable stat to cut power to the boiler, so it won't short cycle. Even
without any additional zone valves at all, you can install a cylinder stat.

The main disadvantages compared to a zone valved system would be:
1. Loss of temperature control of HWC when space heating is called for (but
only when the thermostat is active, not just because CH is on).
2. Possibility of parasitic gravity circulation on heating when only HW
called for.

It still won't comply with Part L1, but it will be massively better than
before, for just the cost of two thermostats and a bit of cable.

Christian.