If you get the non o retirement in the UK, you don't need insurance or a criminal check. It should be fairly straightforward, although my recollection is that the system is a bit clunky when uploading the evidence (it might have improved)just be patient and keep trying.
Once you get the visa, come to Thailand, make sure immigration stamp you in using the non o visa.
Open a bank account in the first few days and deposit the 800,000. Make sure when you transfer the money it shows as an international transfer (if using Wise, I think you use the option to purchase a property ').
Go to your local immigration office and ask for their requirements for a 12 month extension of stay based on retirement.
Get a 6 month lease property with a contract that contains the owners id and their house ownership book.
When you get to to 15 days left of your visa, go to the immigration office with all of the evidence they requested and apply.
Keep a folder with all your documents, photocopies and photos in as you will need the same information regularly in Thailand.
it is illegal to use an agent to circumvent the 800,000 baht financial requirement. If the immigration officer is investigated all of their visas issued illegally are cancelled and you would be deported (yes this does happen).