Sterling is perfectly acceptable. Lots of people think they have to take US dollars abroad, and so they make a loss on the sterling to dollar exchange, then another loss on the dollar to local currency exchange. Of course, banks love this and will advise you to take dollars when it isn't at all necessary.
Save money - stick to sterling and change it to Thai bahts when you get there.
I lost out last year when I exchanged sterling for Bahts in England. I'm taking mostly sterling this year and exchanging it in Thailand. Don't change it at Bangkok airport the rate isn't good.
I would also stick with the good old British Pound. You will probably make an extra 5 or 6p in every pound if you exchange for Thai Bahts when you arrive. It doesn't sound much, but that will really mount up!