You can do an airport upgrade for cash (and indeed now Avios albeit at a poor rate) subject to availability. Ahead of that, you can't do a cash upgrade on a redemption via the website or contact centre.

So your best bet is to hope that4 AUP spots are available on the day. I'm not sure how likely that is on a short haul route, and it may come with a lack of catering since BA seem rather unable to react to short notice bookings, so you might take that into account.