The U.K. based insurers normally have a moratorium and cover pre existing conditions after a period. You have to ask to have it removed. In Thailand I don’t think any offer this but get it before your 70 otherwise self insure.
you can take your “house book”and his details (copy of passport ) to the Immigration office and fill in a paper form and do it in person. Must be done within 24hours or next working day after a public holiday.
Do your due diligence first. I read recently that Wise transfers don’t count as money sent from abroad which is a problem if at some point you need to repatriate it.