Every BT customer's router (a.k.a 'HomeHub') effectively broadcasts TWO signals (unless the customer has changed the default settings, and turned the second one off).
The first one is for the customer's use but the second one is available for anyone using the 'FON' system to connect to. That means that any other BT user (plus people who pay BT to have access to the FON system, such as me, and anyone from abroad who uses an ISP linked to FON) can make use of that signal.
So, because there are millions of BT HomeHubs out there, BT customers (and the others I've mentioned) can connect to the internet almost everywhere in the country via other BT customers HomeHubs. (It's completely secure; the customers own internet traffic is kept completely separate from that of the fellow BT user sitting in a car outside his house).
When connecting to that signal though, the 'visitor' will initially appear to get straight in but then, when he tries to go to any website, he'll find that he's diverted to the BT/Fon log-in screen, where he has to enter his credentials. (It's occasionally possible to get directly to a website but any attempt to proceed further will go to that log-in page).