Can't you just book it and then call up to upgrade with Avios?

Admittedly not as simple but if you a GCH it won't take too long.

The reason for it is that Avios/Ugrade bookings have to be made in your own country, so it's re-pricing it for a UK point-of-sale, and thus you're not getting the Ex-EU 'deal'. However, the common opinion is that they'll still let you upgrade with avios on the Ex-LHR leg if you call up, regardless of whether that's technically correct or not.