There are plenty of currencies exchange booths in BKK, you can find them everywhere in Sukhumvit. I changed money in a a booth nearby Sukhumvit soi 18 merely few weeks ago.

You have roughly 6500-7000 Thai bahts. I don't think this amount is adequate to cover the expenses for 2 persons in 3 days. Anyway you can change Uk pounds at any currencies exchange in the ariport or at the street.