A private briefing cannot over-ride the Conditions of Travel. It is the appropriate First Class fare, which may or may not be the Anytime fare.
I have half a mind to test the matter when I am travelling back from London later this month: it will be either the difference to a First Off-Peak fare or a formal complaint with escalation as necessary.
Although technically, if facilities exist at the station beforehand, are you not obliged to make use of them? If you sit in a first class carriage with a standard ticket, on some operators you can be given a very expensive Penalty Fare or even prosecuted.
Although Avanti don't seem to operate a Penalty Fare scheme, on what basis do you believe you are entitled to upgrade on the train, if facilities were open at say, Euston? Surely the operator is entitled to treat you as travelling without a valid ticket.
National Condition 15.3 says you should upgrade beforehand, before you board the train.
15.4 says some operators allow a supplement to be paid on board, and where this is the case, will be published in notices. Does Avanti have a notice anywhere saying you do not need to upgrade beforehand for first class accomodation?
Edit:
I have actually found such a notice, so unless / until they delete that, you should be OK! If they do remove that text though, it seems they could penalise you legitimately.
Book First Class and you'll get to enjoy our First Class station lounges, complimentary food and drinks service, and seating upgrades.
www.avantiwestcoast.co.uk
Can I upgrade to First Class on the train?
The simplest way to go First Class is to select a First Class ticket when booking. But if you've booked Standard and you feel like upgrading once you're on the train, just speak to the Train Manager. You can pay the difference onboard.