UK bank should do just fine. But make sure your name is visible on the print. I had a spot of bother with that bit. So, I printed the balance screen that has my name on it, and also supplied the ins-and-outs print for the preceding couple of months.
You will not need proof of travel to get the visa. There is a slight risk of being asked to show a ticket within 60 days at immigration when entering Thailand. But, as I'm sure you don't have an extensive entry history here, I wouldn't concern myself.