I think you are a bit unclear on how the "Book with cash upgrade with avios" option works, or at least it reads that way.
If you choose the class as World Traveller, it will upgrade the legs you select (inbound or outbound) to World Traveller Plus.
If you choose the class as World Traveller Plus, it will upgrade the legs you select (inbound or outbound) to Club World.
The booking requires award space in the one higher cabin for the legs you select only, and revenue seat availability on both legs. The upgraded leg will be in an upgraded revenue fare class (WT is Y, B, H).
You selected upgrade inbound only, so it has only found a YVR-LHR award seat. The outbound (LHR-YVR) is a cash/revenue seat.
Note that the BA "Book .. upgrade.." option has a bug, and shows "1 reward seat left" when it should be "1 seat left" if it is not the flight being upgraded. You are seeing that there is one _revenue" seat left in WT on the outbound (LHR-YVR) on 9th October.
If you select this option and search 9th Oct outbound, ~May inbound, select upgrade inbound only, select World Traveller, you will be shown:
A revenue (cash) seat on LHR-YVR
An award seat in World Traveller Plus on YVR-LHR, AND the underlying cheapest eligible WT cash fare (which is Y, B, H).
Hey Nux thanks for your reply and explanation. I did understand it as you describe but then why is it that when i go through the whole booking process i get a total fare for 2 people for 2317 plus the avios whereas if i book just cash in WTP both ways (same deal except in the former the WTP is upgraded to Club with avios) the same fare costs me £3850. The BAEC site is not matching up with BAs own flight search engine. I know this as i tried to book this steal of a deal about 10 times and each time i get rejected
