@Gary *******
>open a bank account and deposit the money and after it's been in the bank for two months I can apply for a retirement visa from inside Thailand
MOST immigration offices allow you to get an in-country Non-O visa.
>If this is the case, which visa would you suggest I get from the UK in order to enter Thailand initially?
I suggest getting a visa that lets you stay as long as possible. I recommend a single entry tourist visa. This visa lets you stay in Thailand for 60 days, and then apply for a 30 day extension of stay based on tourism. This should give you enough time to open a bank account and season the money.