That's interesting. I've played with the evening trains to Stafford and get a similar differential in fares. The itinerary I generated certainly figures the letters CIV quite prominently. The same fare is also available on the Forum's site, but I can't see any mention of CIV in the itinerary. I don't know what the actual tickets will say.
The
Man in Seat 61 says however "Avanti West Coast currently looks as if it sells LNE fares, but it doesn't." I'm probably overthinking at this point.
I think I'm happy buying Advance tickets from London International (CIV) from Avanti. If we are delayed I think there may be more chance of their staff accepting these. I'm also encouraged by the experience of others on Advance tickets when Eurostar has been delayed. Thanks everyone.