What are my options for renewing my marriage visa in Thailand after it has expired?

Apply for a 90 day single entry Non-O visa based on marriage to a thai online using the eVisa website of the thai consulate in London,

OR

come in visa exempt (free entry) then once you're here go to the immigration office apply for first a 90 day Non-O visa based on marriage and then 2 months later a year long extension of stay

Whether you need proof of onward travel flying in free entry (visa exempt) is between you and your air carrier, and if you can board your plane without it you're unlikely to be asked anything at passport control when you stamp in here