Annie456, welcome to flyertalk. Yes, you whould clear immigration in Krabi and it would depend on the flight loads but I dont know how many international flights land in Krabi so it might not be so bad. You should not need a visa on a UK passport. How much time do you have between your scheduled flight landing and the boat departure? Also, if you are checking bags that could make a difference also.

Again, welcome to flyertalk and I am sure there are plenty of people with more experience in Krabi than I that will give better answers to your questions.