HKG airport is super-efficient so 2 hr 50 mins is more than enough time unless your inbound BA flight is delayed. Please note that check-in for Bangkok Airways from HKG to Samui is supposed to be 2 hours before (not 1 hour as someone has written earlier on this thread). PG 806 is the last direct flight of the day from HKG to Samui so if you're delayed and miss it, you'll have to stay in HK overnight or make alternative arrangements.

I assume you're travelling in J to Samui? If you're in Y, make sure you get a seat as near the front of the plane as possible. People at the back arrive latest at Samui Immigration and this can mean over an hour queueing (Immigration is slow and usually only has 2 or 3 desks open and this is for the entire flight). I always book J on PG806 as it's worth the extra money to be out of the airport and in a cab while all of Y is still queueing to get through Immigration.