Julie Jasinska you talk about 60 day or 90 day Visas.
You might be looking at tourist Visas, rather than Non Immigrant Visas.
For retirement, you should get a Non-O Visa based on retirement. This will give you 90 days stay, in Thailand, from the day you arrive.
It will also enable you to open a bank account, where you should deposit the 800,000 Baht, as early as possible, so that it has been there for a minimum of 2 months, before you apply for the 1 year extension of stay.
To qualify for this Visa, in the U.K., you need to have the equivalent of 800,000 Baht in your bank account, or can show that you have a passive income of, at least, the equivalent of 65,000 Baht per month.
However, once in Thailand, you can only qualify for the 1 year extension of stay, if you have 800,000 Baht in a Thai bank account.
The 65,000 Baht monthly income can only be used for subsequent extensions of stay, so, if your intention is to convert to the monthly income, you should commence these monthly transfers as soon as possible, once you are in Thailand.
The monthly transfers can be spent, whenever you want. There is no balance of these amounts that need to be “saved”
I hope that it all goes well for you.