Carl, if you have insurance in the UK already that meets the Thai requiremtn, then all you need to do is take that to the Thai Embassy and show it to them along with the form on the embassy website. They sign it, and you are good to go.
Understand, but still needing a bit of advice. My plan is to get Medical Insurance in Thailand. Which Visa would be better to arrive with in order to get the insurance and then re-apply for a long stay visa?? Or can I just purchase the Thai Medical Insurance online prior to obtaining the OA Visa in Los Angeles.
Under the requirements section, sentence #6, it states Thai or US Bank. For the extension, the actual requirement is that it must be in a Thai account, but some of the offices do accept the US Bank account statements. Better to err on the safe side and open the Thai account.