I've checked ex-EU and the cheapest is about £1700, so still another £600 more than WT+, that's without positioning flights to the EU airport.

It's £400 upgrade WT+ for both outward and return journey, so £200 each way.

Sadly just looked and I'll only have 26,000 Avios by the return leg so not enough for the 35,000 upgrade price. I would also hopefully have a £200 voucher with BA though if an offer came through.