Anyone going back to the UK soon? (Bangkok)

Here's my problem, I brought several hundred pounds of GBP on holiday to BKK. I didn't look at the 20 pound notes, but the forex people here certainly do. The problem is, they are "old series" and they aren't accepted. Back in the UK, no problem.
I will run out of cash and start getting advances on my credit card next week some time but if anyone either has a suggestion for a place to change them to Baht, or.... I will take 10% under the rate for changing them to Baht. Anything more than that, I'll just go with the credit card. Meeting only in the lobby of a bank or somewhere safe.
Mike