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[perksie Level 60: Scholar]

Some BT sites are Fon/Openzone, you cannot connect to these, see the list of available hotspots in the link above.

[Keyboard_Warrio]

BT Fon hotspots are often incorrectly picked up as 'BT OpenZone' and I don't believe the o2 connection manager can distinguish between the two. This is a problem with how BT identify their WiFi network(s) and not with the o2 connection manager.
HOWEVER, it does sound like you are connecting to a proper BT OpenZone since you get to the o2 landing page. Text 'WIFI' to 2121 from the connection manager software and this will reset your dongles mac address with BT OpenZone and may let you get access. This can take up to 24 hours to reset though!
Only thing I can suggest...

[perksie Level 60: Scholar]

The reset should only take half an hour.

[Keyboard_Warrio]

The reset should only take half an hour.

It shouldn't take longer than about an hour but I believe most o2 advisors will quote up to 24 hours. I doubt it ever takes that long but I think it's just one of those "just in case..." things.