To make up for this we are thinking of replacing the bathroom
radiator with an electrically assisted towel rail - so that
it can be switched on when the heating is off.

You can run it off the unzoned pipework instead of the heating zone. This
way it will get hot when the hot water is heated, or when the heating is on.
This is good, because you often want a warm towel after a bath. Running the
bath makes the radiator hot as the cylinder is reheated.

If you do go electric, ensure it is fed via a 30mA quick acting RCD. If
there isn't already one on the circuit you are proposing to use, install an
RCD fused spur.

Christian.