Nope. If the return cools the house has dropped in temp and/or the TVRs
have opened.

********.

Say the outside temperature is 30C. The rads require no heat, therefore the
return temp drops below 71C. What prevents the boiler firing?

The answer is nothing. The boiler will fire. It will heat the primary
circuit, wasting energy and it WILL NOT comply with Part L1.

There is nothing in your scheme that prevents the boiler firing if the house
is hot. OK, there is something to stop the boiler firing if the house is hot
AND the system is already running. However, if the system has stopped and
cooled down, then it will restart, cycling forever, despite there being no
call for heat at all.

Christian.