OK, I think I know what's happening - if you do a search on google flights or Matrix, you will get the £973 fare, clicking through to the BA site tells me it's selling class T. If you do the same on BA's site, with an UUA as part of the booking it comes out as £1,503 + 60,000 Avios. What I can't tell is what selling class the latter is, although it's clearly different.

What you need someone more expert than me to say is whether or not a T fare can be UUA - if the answer is yes, and assuming there is still the U availability when you come to book, then I'd book the £973 fare and immediately upgrade once ticketed.