Asking this question seems to bring out the best in everyone, probably second only to the hotel question. I fly from Manchester every time and have been with a few different airlines,
BA do a codeshare with Qantas and this is my favourite way to go; shuttle to London Heathrow in around an hour and 12 hours or so to Bangkok.
Emirates will take you via Dubai if you have no hang ups about transiting the Middle East. Never done it myself but both legs are around 7 hours, so no time difference.
I tend to budget around 550/600 gbp for flights, but there are bargains to be had for the 300 mark just now, though most seem to be out of London Gatwick.
Trailfinders in Manchester are the biz for scheduled flights. Try
www.westeasttravel.com for bargains.