Actually, thinking about it, it can't be compliant, as the bathroom
radiator without the TRV would mean that any such flow switch would
be ineffective.

But then, I suppose you could put the switch after the bathroom rad. I would
imagine such a system would be compliant.

BOILER ----+-----FS----+----+
| | | |
| RAD TRAD TRAD
| | | |
+----------+-----------+----+

RAD = bathroom radiator/bypass
FS = flow switch interlock
TRAD = TRVed radiator
= pump, runs continuously, or pulses occasionally when FS off

The bathroom radiator would only be on if another radiator in the system
required heat.

Christian.