hi . I can't really say as I used Chang Rai immigration . I was staying with a friend and he registered me on TM30 , he came to immigration with me and showed his rental agreement. The letter was 500 baht .
yes I did everything in Thailand. I arrived on a tourist visa .
First thing I tried to do was open bank account , especially as I was under the believe the money needed to be in the account for 2 months before the non O application . It turned out that wasn't the case , you just had to show a 800,000 baht balance within 24 hours of applying .
you would probably benefit from opening a WISE account while in the UK. This will give you a easy way to transfer your funds (800,000) into a Thai bank account.
As I mentioned before different immigration offices required different documents. You may find that you only need TM30 registration which is done by the landlord, you then just make a copy of it .
For me in koh phangan this is what was required . Yes you will need to be in Thailand and you will need to open a bank account , believe it or not that is the most difficult thing to do . I would suggest you just take it in your stride and slowly but surely you will get it done . I wouldn't worry about the funds being in the account for two months prior to applying as I did not have this demand , although you will hear it from many people.
I would suggest you enter Thailand on a TR visa and with the extension this should give you enough time to get everything sorted .