Yes, BT Fon is visible as a wifi network in lots of places across the UK, almost any BT Broadband customer will have this being provided via their router, and they too can use BT Fon when they are out with laptop or mobile phone (though I think restricted to customers of level 2 and 3).
If you visit the BT broadband website or search for BT Fon there's a coverage map (I found a dozen or more networks on the map for my local (small village) area.