Originally Posted by corporate-wage-slave

There will be 2 refund mechanisms open to you, and you need to do the sums to find out which is best.

BA will simply refund the POUG on request, and that should be straightforward. I suspect you need to complete travel first though. (AUP is done at the airport on the day of departure).

Mennens - see the wiki - is also applicable if you were ticketed into WTP and end up in Virgin's main cabin. Now that is definitely only claimable after travel since it's a complex calculation but the wiki has some worked examples. It's just possible that the amount payable will be similar to the first reimbursement since Mennens is adversely weighted with APD, whereas there was no APD component visible in the POUG. If it's similar go with the first one since Mennens is often a bit of a mess to claim.

Usually, though, I would expect BA to maintain your ticketed cabin given that Virgin have a very similar product to WTP. Certainly if being rebooked BA to BA you would have stayed in WTP. So it may be worth a telephone call to see if VS' Premium Economy product is open to you.