Now I used the “upgrade using Avios” booking form to see the price for booking into Premium Economy and immediately upgrading using Avios, for the same March dates as I checked for a straight cash booking. The cost came to £1567 plus 60K Avios per ticket. Those are “off-peak” dates so the Avios may be different on your actual dates. And of course the low Club World all-cash price may be different as well.