I appreciate we are all about miles on these fora, but I would recommend that if possible you ignore Travelex altogether and just get your cash on the spot in Thailand. The rates that Travelex offer are simply awful. Currently showing a bid-offer spread of basically 70-80 for GBP/THB. Seeing as the interbank rate is about 74, that's one helluva fat margin.

If your bank's cash cards use the "Plus" network, there are 3 or 4 ATMs in the BKK terminal - just hang a left as you get into the int'l arrivals hall and they are on the left hand wall about 25 yards up. I've used my (HK based) "Plus" cash card everywhere from the UK to Phuket without problems. Oh, didn't work in Bahrain though...

There again, maybe UK banks will rip you off on the rate they use to debit your account...