Welcome to Flyertalk and welcome to the BA forum.

Yes you first need to find redemption space in your new cabin. And in broad terms the UuA amount is the difference between CW and WTP if bought as a redemption. The "plus taxes" bit is true, but the big difference is the difference in carrier surcharge which could be substantial on that route, therefore do ask BAH to see what the upgrade fare for cash would be too, just in case it's better value. As a very, very general guide, your 80k Avios is worth at least £800 if you have an open hand - most of us don't have total flexibility though.