An agent in the UK can only apply for the initial non Immigrant O visa, which is very simple to do yourself on the Thai e-visa website, providing you can show the required documents, I recently heard a UK agent quoting £250+vat, it's £60 for the initial non O, do it yourself, then when you arrive in Thailand you will be given a 90 day stamp, then go to the immigration office you will be using and get a list of requirements for a 12 month extension based on retirement, you have then got plenty of time to get all the required documents together, either that or get a 60 day tourist visa, from the Thai e-visa site, and contact a visa agent as soon as you arrive in Thailand, and they will assist you with the whole process.
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