Maybe squawk about the sale of goods act? Not sure if it applies to air tickets, but basically the price on a shelf is an offer to sell, and the if the transaction is accepted by a retailer (in a physical store at least) then it must be at the same price as displayed. It's illegal to charge a higher price - that's why if you ever find an error, the supermarkets will usually give you double the difference back...they have a gazillion labels to change and it's inevitable that some get overlooked and don't match wot-the-computer-says.

I see this situation as analogous. You had a ticket. You requested ba.com to offer a selection of changes. Some had prices, some did not. You selected one of the choice offered and completed your transaction.

There is something in contract law that says a vendor is not obliged to sell (he can refuse a transaction), but I don't remember the details. Pretty certain it's only before the transaction is completed though....which I would interpret in an online context as "interaction between the ticketholder and ba.com has finished. The booking is changed and payment (nil) processed; the order (ticket) is displayed in ba.com as selected and without reservation such as pending" Certainly would not regard 3 days later as acceptable.

But as said....no idea if this applies to air tickets.