For Non-OA, specific insurance is required. I don't think you need a return ticket, but better to check with the embassy/consulate where you are applying at. Exactly detailed requirements are consulate specific.
Applying on the eVisa system means all visa issuing process is completed online. You will get your eVisa receipt as a PDF file sent to your registered email address. No need to send the original passport by post.
The visa is valid for 1 year to enter Thailand on it from the issued date by the embassy/consulate. So it'd be reasonable to apply for it near to your departure date (but not too near in case it takes longer).
You can get an account on eVisa system, and go through the process to see what you need to prepare.
Please try to check the website of the Thai Embassy, London for requirements for Non-OA, too.
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