A by-pass loop is totally different from a by-pass valve. An
unrestricted flow of heated water from the boiler must be allowed
to travel through at least one radiator circuit to prevent the
boiler from over heating in the event that a fault occurs.

But your interpretation bans the use of zone valves. When the room
thermostat is satisfied and the zone valve closes, where's your bypass loop
now? It needs some sort of automatic bypass loop to take the water as the
boiler pump overrun happens. Losing 10-15C over an automatic bypass loop
does seem somewhat inachievable, and probably illegal. Of course, the boiler
should have shut down as the zone valves started to move, so it shouldn't be
adding more heat to the circuit.

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.

Christian.