Easy enough to get a single entry Non O in the UK for retirement provided you can supply the documents they ask for.
Not forgetting you'll need to apply by the London Thai Embassy E visa website.
You can use a consulate at Liverpool, Hull, Edinburgh, Cardiff or Dublin but you need to email your documents for pre approval from London then make an appointment. There is no postal services via the consulates in the UK or Ireland.
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Non-Immigrant Type O (Retirement (pensioner aged 50 or above with a state pension who wants to stay in Thailand for no longer than 90 days) - single entry only)
Financial evidence e.g. A copy of pension statement if the applicant is a pensioner, or a copy of 1-month bank statement showing your income from pension, or 3-month bank statement of at least £10,000
Confirmation of legal residence in the UK or Ireland