Just came to Thailand a few days ago, similar situation, flew in together with my wife.
I just want to confirm, indeed you need to choose 'Non-O Stay with Thai Family (more than 60 days)".
My embassy (not UK) later (after the initial application) requested proof of return / onward flight, which was then again requested by the airline before embarking on the flight, so I'd strongly recommend you use a reservation such as onwardticket so you can show it in case it's needed. I had something similar.
How the process works is a bit different with each embassy as I understand it, mine took almost 2 months to process my visa, but I've read the UK one is quite fast.
The embassy also (additionally) asked me for a letter of invitation and proof of ownership. Quote: "Please upload to your account: Letter of invitation + Inviting person's house proof of ownership + the Thai ID card."
In my case I had the 400k in the Thai account for more than 2 months when I applied so I'm not sure whether that'll be an issue or not.