Welcome to FT,
magnagraecia!
You may want to have a look at our dedicated thread about special offer upgrades that you may see when you look at your booking on the BA website. This will give you an idea of what prices these offers might be pitched at.
Pro-active Online Upgrade (POUG) tracking and discussion | 2019 thread
A special offer of this kind will be marked as such. Otherwise, what the website will quote you is the upgrade price in accordance with your fare rules, which will be the fare difference to buy up to the higher cabin plus the change fee specified by the fare rules. I suspect that the £269 price was probably one of these.
Beyond that, I don't know whether there's any reliable way of finding out what your chances are of getting an offer.