I'm looking to fly London-Vancouver, and then Toronto - London, buying WT+ and then MFUing both legs into Club. (I'll buy the Vancouver-Toronto flight separately, BA doesn't seem to want to sell me that leg)

I can see availability for the WT+ -> CW in both directions, but no way to book online, AFAICS the book with money and upgrade with Avios form seems to assume a simple return.

So is it as simple as check the MFU or even pure Avios availability (these are the same I believe?) for the dates I want, and then phone to book? (waiting for the sale in a couple of days of course)

I could just book and then MFU but dont want to take the small chance that between booking and MFUing the availability goes, I am very flexible on dates.

TIA