The pipework does seem to be quite well insulated but I don't suppose
this is relevant to the situation where all TRVs are shut and the only
flow is through the internal bypass in the boiler.

The problem comes when the radiators are mostly shut off. In this situation,
you would probably want the flow switch to cut the power, as the boiler will
be well below modulating minimum and will short cycle. Better to cut totally
and wait until the rooms require real levels of heat.

Without the interlock, enough water may leak past the TRVs to keep the
primary circuit hot at all times, with the short cycling boiler, so you
really need the entire circuit insulated.

Christian.