Over the last month the mid market bank rate has gone from 42.6 to 44.0 to the pound. What your getting at a currency exchange sounds good to me. The gamble is whether it keeps getting better or drops again.
from what I've seen of it, it is hard work to get the STV with all its restrictions. Surely easier to get the tourist visa, and then get the extra 30 days in Thailand. That's only my opinion and I may be wrong.
When I applied for exactly the same at the end of last year there were two options. As you say you could either show proof of pension(which I didn't have at the time) or you could show 3 months of bank statements showing in excess of £10,000. This was last November at the UK embassy. But it may have changed now.