British Airways has announced a codeshare arrangement with Bangkok Airways. This will allow you to earn Avios points on selected flights with the Asian carrier.
The following (brightly coloured, see photo!) flights will now carry a BA flight number:
Bangkok to Koh Samui
Bangkok to Chiang Mai
Bangkok to Phuket
Singapore to Koh Samui
You are now able to buy through-tickets on ba.com to any of these destinations with luggage automatically transferred through (and your British Airways baggage allowance carried over). Avios points will be earned at the standard rate for short flights – it is not clear yet if the flights will also earn tier points.
Standalone internal Thailand flights on these routes are also bookable via ba.com.
You cannot redeem Avios for flights on Bangkok Airways, unfortunately.
(PS. Horris at Flyertalk pointed out that if you are flying Bangkok Airways, you can get an extra 10kg baggage allowance by joining their free frequent flyer programme.)