EDITED as I did not know you had a multiple Entry Visa: The visa would be good for 6 months -- that means you have 6 months before you use it . When you arrive in Thailand you are given 60 days you can extend this by 30 days (fee 1,900 baht) but then will need to go out and re-enter Thailand and get another 60 days which is extendable by another 30 days !
I thought that for an O class you do not need to bring any money in! just apply in the UK show an equivalent of 800,000 baht! But if you want a retirement visa then you have to bring in 800,000!
My gf just arrived having had problems trying to get a 60 day visa in the Thai Embassy in Cebu, Philippines. It was not denied but the officer just gave her the run around, and ended up with NO Visa so she arrived hoping to get 30 day visa exempt. This is her 3rd one this year (71 days in Thailand this year up to today) She was not asked 1 question, and breezed through immigration (Don Muang) Although she was prepared with $600 cash, a hotel Booking, return flight letter of support from me. All's well that ends well!