Uk insurance will only cover a fixed period, you need to apply for Thai annual cover on arrival. Most won't cover existing issues for a period of time, one or 2 years, some want list of medications and a full medical. Lots of choice but depends on age
most generally need certificate of residence which you get from immigration. Many are here to retire so no work permit, renting a condo so no yellow book
i fixed my funds last year and currently getting 5.5% with HSBC OFFSHORE Friends opened account in Cambodia with May bank gets 10%. You can shop around, try Cherry Godfrey isle of Mann also