You can as you probably know connect free to any of these while in credit. Some hotspots are BT Fon, and many are privately owned, but still show as BT Openzone. O2 are also setting up free wi-fi for everyone later this year:
Aug 19, 2014 Connecting to Free BT Wifi - YouTube Jun 22, 2015 How to connect to BT Wi-Fi hotspots using your smartphone
If your price plan includes BT Wi-fi, you'll be able to use thousands of BT Openzone hotspots to browse the internet, read and send emails, stream music and watch videos while you're out and about. It won’t use your mobile data allowance but there’s a 3GB monthly limit, part …
BA Lounge – How to connect to BT Openzone Free Wi-Fi service
When you connect to a bt openzone, the o2 advisor is correct. It will take you either to payment page or to a mobile zone page. Ive connected to a few where i live and it comes up asking me which network im on. I choose o2 and thats it. If o2 isnt listed or you choose o2, it will say that you cant use this service which means that bt openzone
BT provides Wi-Fi in public places (hotspots) such as airports, hotels, coffee shops, motorway service stations and city centres. This includes Hilton Worldwide hotels across the UK and Ireland so you can connect to the internet during your stay.