I know it is pretty useless me replying to this a year later but someone else may need the answer.
Your just log onto BT wifi from your iPhone or whatever at htttp://btopenzone.com:8443
The pi has OPENWRT installed - don't try without. It works but is very slow (well was for me).
The Pi is connected to the BT hotspot as a wifi client (DHCP) on wlan0 but without login.
The OPENWRT RELAYD package bridges wlan0 and eth0 (don't try a wlan-to-wlan bridge, its a nightmare).
How to set this up is on Youtube : Use this method
https://www.youtube.com/watch?v=-Vi91fTqsEUThe wifi router attached to the Pi as it's eth0 client via ethernet cable and fixed IP.
[I use a Fon router]
Your phone/tablet is the wifi client of the router (DHCP).
You have to make sure that the house router and the BT wifi hotspot are on very different channels.
You do not have to specify the MAC but you can easily do so via OPENWRT's Luci webinterface. It automatically updates wpa_supplicant and the other config files. You can also edit them manually via Putty or whatever as per normal Raspberry Pi work.
The reason you may want to specify the MAC is because with a good aerial, you might be able to access 20+ networks called BTWiifi-with-Fon and the best one won'tr necssarfily be the one with the strongest signal
(Imagine 3 networks, all strong and on channel 6; one slightly weaker but fine on channel 11; those 3 may be too weak / far-apart to interfere with each other but could really confuse your mega-aerial!!).
Hope this helps someone out there.
I am working away a lot in digs (with no wifi) at the moment and am using a similar set up. 20MBPS, no problem. BT login only falls over every 24h or so. Don't believe all this talk of speed caps on BT Wifi - if there is its pretty high unless you are an online gamer etc.
Hope this is useful to someone.