I still don't understand the objection to adding a simple flow switch to the
radiator circuit. It costs very little, makes the system explicitly
compliant with approved document L1, is simple to wire and prevents your
primary circuit being hot 24/7/365. (Unless you sit by the controls and act
as a manual room stat, turning your system on when it gets cold outside).

I didn't say that it was optional.

However, some of the methods described as "best practice" fade
into inignificance if a properly designed modulating boiler is
used.

I read the document as being suggestions of what might be considered
reasonable provision, rather than best practice. You are required to make
reasonable provision. If it considers system 'A' explicitly with feature 'B'
to be reasonable, then I can't see how system 'A' without feature 'B' or an
alternative to it can be considered reasonable.

The system without this feature depends on user intervention to remain
efficient. This appropriate user intervention may not be apparent when you
sell the house to someone else. It relies on the user to understand that the
system uses considerable energy even when the house is not being heated.

Christian.