I didn't really expect it to work, hence saying "alas, it's an Avios ticket". Was mostly posting as a new data point for KEF since i assume the price is what would be available on a cash ticket.
All that said, the
upgrade Ts & Cs do explicitly say
"If you purchase a promotional upgrade from a redemption ticket, you are not eligible for tier points in either the original or upgraded cabin.", which implies that such an upgrade is occasionally possible, Maybe that line is there to cover BA's back in the case that their IT screws up so badly that the purchase goes through? 
I've yet to see one go through.
I've got a £110 per person upgrade on a reward flight to Milan but throws up the usual error.
It's kind of wild though - BA aren't letting me give them the £110 even if I wanted to!