I am in the same position.
Having used the outbound portion of my upgrade voucher to move from CW to F, months ago, I have been trying almost daily, to use the return portion from ATL to LHR.
The return flight in 2 days away and 5 seats available in First (costing $10K to purchase) but nothing available…..
For the record – I have never been able to use the return portion of a Gold upgrade voucher from CW into F.
Outbound can be found but (in my case) never anything on the return.