BT Wi-fi lets you connect
(a) to your own Home Hub (which has to be reasonably close to the iPad) ; or
(b) to any other BT user's Home Hub (assuming that they've not disabled the facility) or other BT wi-fi hotspot. Instructions for doing that are here:
http://www.btwifi.co.uk/help/login-with-bt/ (NB: You need to read BOTH the 'connect' AND the 'log in' parts). However, once again, the Home Hub (or other BT Wi-fi hotspot) you're using must be reasonably close to the iPad. So, for example, you can usually find a house (or business) with a Home Hub functioning as a wi-fi hotspot in most streets but
you need to stay close to it to maintain the connection.
So you can't use BT's own wi-fi on the move in a car. The only way you can get connected is to use the mobile phone network (which can be quite expensive). The device in Baldric's link connects to mobile phone masts and then creates a wi-fi hotspot within your car (so that several passengers could connect to it at once if they wanted to). So you have to buy the device AND pay for the data you download.