On the Avanti I was on, the guard said that they were accepting LNR tickets but only for standard class and they wouldn'tbe declassifying first. I think his point was basically not to push it!
Perhaps this is a general Avanti 'acceptance' restriction because of course no-one can have an LNR first class ticket.
I might even have some sympathy with Avanti for this restriction. Admittedly some time ago now I boarded a Euston train at Piccadilly to find it full to almost standing in the First. I later found out this was because Avanti had given ticket acceptance to EMR ticket holders travelling from Sheffield.
What happened was that all these transferred passengers made such good use of the Avanti First Class menu and offers that, by the time they reached my coach (the then-unreserved nearest Standard) the crew had run out of everything including hot drinks and we received no service.
Might sound fair enough; but remember, the EMR offer in First was coffee in a paper cup and choice of a packet of 2 Walker's shortbread or 2 Biscoff biscuits. Possibly a sandwich in the evening peak. Did seem that people were exploiting Avanti after being helped out by them.