This is possibly a real PITA for them tbh. After all the only place now that these can be exchanged is the BoE. While I'm sure such an organisation would have back channels with the bank possibly it probably would be a bit of annoyance to process. At the very least they would need to sort the notes and get them back to the UK to deal with. (though of course a 20 or 50 quid note is worth a coughload of, say, HK coinage of course)
just a thought.
The more recently phased out notes can be exchanged deposited at banks or via PO still, and given that EasyJet is a participant in the program, I am sure this is something they are already dealing with. I wouldn't think that having $100,000 worth of Thai Baht in coins is not the easiest thing to deal with either. You suggest, "At the very least they would need to sort the notes and get them back to the UK to deal with", isn't what they have to do that with most coinage anyway (sort them and get them back to the country of origin), or do banks in Australia (for QF collections for the scheme) allow deposits of a wide range of foreign coins?