This is to preserve the value of the premium cabins, the idea being that if you can ALWAYS upgrade yourself on a space available basis on the day of flying, many of those with status (i.e. those high on any wait list or preference order) might decide to chance booking the lower cabin then upgrading on the day.
So with BA you can't upgrade with miles on the day at the airport - I'm not sure what the cut off is though - but before, even with seats available, these may not be released for mileage receptions. You just simply have to call and see what the score is, definitely before you check in. Sadly BA don't even offer a wait list, so you have to keep checking.
The cash upgrade option option and cost values too. On many occasions its simply the difference between the cost of your ticket and the cost of a fully flex ticket in the next cabin - i.e. A lot! But this does depend on route and cabin, and is occasionally offered on a fixed cost. SO its always worth asking if your route has been selected that day. Y to W upgrades are much more freely avaialble on the day on this basis. Around £250. W to J is up to £500 as is J to F. But these can be less too.
So despite BA not running in a 'full cabin' basis for the premium cabins (i.e. upgrade after check-in close till each cabin is full) - they have been playing around with upgrade opportunities - you just have to be on the right route at the right time.