What should I do regarding my return ticket while applying for a retirement visa in Thailand?

By far the easiest thing for you to do is to get a year-long, multi-entry Non-Immigrant Type O-A (Long Stay) visa from the thai consulate in your country BEFORE you wing your way here.

Here's something I wrote that goes over the three things commonly referred to as a "retirement visa".

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We are confusing the three times you're asked for proof of onward travel. 1 - is by your airline when you check in for your flight if you're coming here without a valid visa in your passport (entering visa exempt). MANY airlines require you have proof of onward travel because if they let you board the plane and you're denied entry to thailand they have to lug you back on their dime. If you have a valid visa in your passport they will not ask you for proof of onward travel when you check in.

2 - is by a thai consulate when you are buying a tourist visas.

3 - is the rare times you're asked for proof of onward travel by the immigration officers at passport control when you stamp into the country.

Most people are ONLY asked for that if they have an extensive history of living here on back to back visa exempt or tourist visa entries.

I don’t think you can book one way. I had a flight booked out of Thailand to Vietnam within 30 days, but paid to have option to cancel the flight and get a refund, which I did. I used agent in Bangkok but with hindsight they were expensive, but they were efficient

Why not get the O-A visa in the UK. This will give you multiple entry and if worked right two years before you need to apply for an extension. Also what you are asking isn't a customs issue it is immigration.

You need a 3 month non o visa. Easy in uk.