Part L does not allow any heating appliance to be uncontrolled. Every heat
outlet has to have either a TRV or thermally controlled zone valve in its
circuit, so can't be relied on as a bypass.

Thinking about it, it could be achieved by using the automatic bypass valve
to bypass a zone valve. You could use the hot water cylinder circuit (unless
it is unvented) or a heating circuit (assuming there is at least one
radiator physically locked on with no user controls).

Christian.