BT OpenZone has been rebranded as BT Wi-fi, so you obviously need to be a BT Wi-fi customer in order to access (what was formerly known as) BT OpenZone. I pay £6 per month for 500 minutes access (with additional time being chargeable if I go over that limit) but I can't see that option currently available to new subscribers on BT's website.
Remember though that there are now lots of places offering free wi-fi anyway, such as all large railway stations, all Wetherspoon's pubs, larger Tesco and Asda stores, all branches of McDonalds, etc, etc.