it's like a cover fee to get into a club/bar, a fee you must pay for any trip you get insured care, in the US. With some insurance that might be $5 - 10 each visit. With other insurance it might be 200 - $1,000. Most plans have capped deductibles, so you only pay them till you reach your cap, say 1,000 - $8,000. It's always a good idea to compare different company's rates each, and every, year as new insurers are getting into the system and might have better rates.
Just fly in, as you're on UK passports, you'll be "stamped in" on your passport for 60 days, NO Visa required. Then a few day before your stamp expires you'll all need to go to an immigration office where you'll pay 1900 baht each to be allowed to stay for an additional 30 days for a total of 90 days. You need to check the dates that are stamped in your passport BEFORE you move away from the Immigration officer, as they are human and sometimes make mistakes.