What is the best visa to apply for if I'm a UK resident staying in Hua Hin on a tourist visa and flying in and out for business?

@Kev ******

assuming you have absolutely no work related activity inside Thailand at all why not look at a Non-O visa for retirement. You can get a 90 day Non-O in country and then extend this every 12 months. You'd also need the multiple re-entry permit along side it to keep each extension alive when you exit Thailand.

Your local immigration office should have a hand out detailing requirements