I think that you will be ok with checking your luggage straight through. We have flown with them a couple of times and I am sure that we have. surprised about BA we have friends that always fly with them and i am sure that they always do.
As for the drink can't help there but if you think there is going to be a problem then buy it at makro or tesco lotus when you get there.
You could try the bangkok airways web site email them. I did once about something and they came back pretty quick.
I am green with envy - have a really good time. 96 days until i go to Cuba ! I am not counting someone else is !!!! I would be if it was Samui.

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Kiss
God laughs at people who make plans.