Building regs require that you have a room thermostat and at least
TRV's on all radiators in sleeping areas (of course you can have
more).

Actually, much as I hate to say it, IMM is correct here.

The regulations don't specify that TRVs are used at all. The requirements
are that the rooms are individually controlled, which could be TRVs or a
separate zone with room programmer/thermostat.

They also state quite clearly that the use or TRV's alone are not
sufficient.

The regulations state that boiler interlock is required. Although
traditionally done using a room thermostat that controls the boiler
directly, it is not a requirement to do it this way. A flow switch on an
automatic bypass loop, for example, is perfectly adequete and compliant to
turn the boiler off when all the TRVs close.

Obviously, TRVs alone are not enough, as you say. However, TRVs with a
bypass loop and flow switch are. As long as the boiler switches off when the
heating load is satisfied, the building regulations requirement is met.

Some quotes of Approved Document L1:

1.38

Temperature control could be effected by room thermostats and/or
thermostatic radiator valves or any other suitable temperature sensing
devices, together
with appropriate control devices.

1.41 (b)
Where it is proposed to effect control by thermostatic radiator valves, a
room thermostat (or other device such as a flow switch) should also be
provided to switch off the boiler when there is no demand for heating or hot
water.

Christian.