Hi,
I have been to Thailand and Hongkong several times,i think the best option would be to fly with the new budget airline oasis ,from gatwick to hongkong, then fly from macau with air asia to Bangkok.From bangkok you can fly to Krabi.
It is really easy to get from hongkong to macau.This would be the cheapest way to go.
My daughter and i are flying with oasis to Hongkong in sept, only cost £520 return for us both.
Airasia are very good ,have flown with them lots of times,with no problems.
There are no budget airlines that fly from Hongkong to Bangkok as far as i know,as we wanted to do this trip before, and we ended up going to singapore instead of hongkong,as this was alot cheaper,but this was a couple of years ago.
Anyway,Hongkong is brilliant,as is Krabi,so have a great time where ever you go
suzi