As far as I know BT Retail don't offer a static IP address. Only BT Business, and even then I'm not sure.
BT Retail won't provide a static IP even when you offer to pay a monthly fee for it.
I was however told to speak to BT Business about it, due to they offer static IP(s) for a monthly fee, but due to there not being a price listed online for a Business version of Infinity 4, I just gave up.
Paul