One problem is that the add-ons would need to be cheaper than buying a new complete goodybag. Especially if they may only last for a couple of days before the original expiry date. The transaction costs make this impractical, though maybe cheaper add-ons could be bought only from existing credit? This gets more complicated if the recurring instruction needs to be cancelled to allow use of airtime credit for a transaction.
PAYG on airtime credit seems the simplest way to do an add-on, where it costs nothing if you don't use it all. No wastage, and the left-over credit remains available to use if you run out the following month too.
The problem is that the current PAYG rates are far too high when you still have an unexpired goodybag.
2p/MB (=£20/GB) is a bit too high, but 15p/minute (=£20 for 133 minutes) is far too high.
When you have a goodybag plan that is unexpired, it would be better if we could match the Three PAYG 3-2-1 pricing.
@krayzt I believe the £12 reduction from 3GB to 2GB is the only time that allowances have been reduced, other than the obvious cases of the removal of unlimited data from various goodybags. (This did not affect me as I could never buy unlimited data as I need to be able to tether.)