I’ve just checked in for the return legs and the upgrade prices are £99 and £69 respectively. At the moment the exit row seat next to me is empty on both flights so I’m not going to take the upgrade, but I’ll check again later.

I don’t expect it is like this all the time when booking last minute, but for these flights the economy + upgrade for all four legs would be nearly £1,000 less than buying business class flights in the first place.