I've noticed this £35pp fee is being forced more and more.
Recently I had a 2 pax 1 way Avios booking (using a 2-4-1), adding an Avios return by phone I was charged the change fee even though the fare had to go to ticketing to work out the taxes and fees.
Also.... the agent said they would normally have to charge a £15pp 'Avios use fee' but this was waived as I am a GCH. Never heard of that one.
Guess I'll have to get back to them on the 2x£35