Thanks for all the responses!

I checked on the BA WT+ itinerary and our corporate TA insisted that WT+ was not upgradeable - did not check any further with BA directly. Further to that, my CC points take a couple of weeks to be transferred.

But most important was the tight connection schedule at LHR when coming back. Same Terminal 5, but I wanted to take some time to buy wine at duty free and can't afford to miss the LHR-HKG leg due to any potential delays. The Venetian room rate for Hed Kandi pool party night are way more expensive than the 110% mileage accrual. And as outlined earlier by another poster, I don't consider WT+ as much better than CX Y.

However, I think I can forget about any CX op-up to J. I checked KVS Tool yesterday night and J appears to be quite full...