The FON network is an interesting approach to internet access...
ISPs who are partners with FON, including BT (Total Broadband) have an arrangement whereby your WiFi router is set up to have two wireless networks with different SSIDs. One is your private network and one is set up for other members of the FON network to connect to. They then open their browser and login using their ISP details.
Basically each customer router becomes a public hotspot. The network traffic is kept entirely seperate from the users and any public usage does not count towards the users usage limit.
On BT home hubs if your a member of BT FON then your hub is updated remotely and you'll find your hosting an Openzone hotspot.