The post says Thai and non Thai nationals from UK so I ask why are non nationals of Thailand being allowed to come from the UK yet other nations that are in better situations not being allowed. The post does not say anything about any specific pre requirements for non Thai .Just says non citizens who need to return.
You should not do anything before about a month prior to departure, also you will need to have a police check done. Am in a similar boat ,was about to pay the money for health insurance when the border was closed .You can contact your local Thai Embassy to get a list of their preferred insurance companies.