Your plan is fine (it's a 90 day visa then a 12 month extension) and only the onward ticket could be an issue, solved easily by buying a cheap throwaway ticket out of Thailand within 30 days of your departure. It's likely less than 50/50 you'll even be asked but you can't be certain (it would be at check-in in the UK, not at immigration). Alternatively you could get the 90 day non-O visa before leaving, you have the finances in place already but possibly might run into a requirement for health insurance for that which you may or may not have in place.