Yes. You are correct. The individual valves can be locked after all.
Yes they might take it off the rad if cold, or worse still, use electric heater!
After doing more research, it seems the Evohome is the perfect solution. However, the cost is about £300 to £400 and the savings might not be as much as I first thought.
I am finding that energy consumption of GCH is not as simple as with electric heaters. Switching off half the radiators will still use much more than half the gas because the cold rooms will draw heat out. As I am learning, the second issue is, say only one radiator is open, the boiler is too powerful and will start and stop and not work efficiently unless there are enough rads open to run constantly and with enough heat drop to condense.
I might have to compromise granularity of control for the boiler to run efficiently and without getting worn out by too much on off.
With all that, I might only save £80 per year best case with Evohome versus the whole house timer thermostat as there is a lot of overlap during room occupied times.
I have found there are valves with buttons on that can be set with 2 high and 2 low periods per day for less than £10 each. With the basic timer thermostat, I can also turn the system off, so have 3 settings (hi,lo,off). Taking into account I will need to have more rads open than I want to keep the boiler efficient, these individual trvs will still probably save £50 per year compared with about £80 with the honeywell and it is not worth an extra £350 to save an extra £20 or £30 per year.
I will keep an eye out for a second hand evohome as it might be worth it for the convenience of being able to instantly change each rad from armchair.
I think a few manual timer trvs will do well enough and I can fit and forget and not waste hours fiddling with the control panel.
What do others here think? Does a fancy system like Evohome cost more than it saves for a small house?
Any tips for planning which rooms are on/off to help the boiler work more efficiently with less wear?