I'm afraid it's all pretty complicated. Firstly it tends whether you mean BTWifi-X or EE WiFi-X. I'll start with the method for BT WiFi-named hotspots (but these are getting rarer every day) and then move on to what to do with the new EE ones.
BTWifi-X was, as you may have discovered, BT's way of letting iPhones auto-connect to BT WiFi. It provided, and still partially provides, a secure encrypted automatic way to use BT WiFi. It has been depricated, and you can now auto-connect to BT WiFi on iOS or Android with the BT Wi-Fi app without WiFi X being used, just by enabling an option in the app. But I do still use WiFi X, as its great. To use it, though, you need an out-of-date version of the BT WiFi app on your iPhone as the latest version has the option removed as BT don't want people using it anymore. The network is also being removed countrywide and is appearing less and less, so I recommend not putting too much time and energy into getting yourself a connection to it, even though its really cool. Your only chance would be if you're a BT broadband customer and already have the BT WiFi app installed and haven't updated it recently. In that case open it and go to More > Settings > Install connection profile, log in again and follow the steps. However if you can't do this you can still auto-connect anyway without WiFi X being involved if you enable the option within the app. Through a combination of the official method and WiFi X I auto-connect to almost every BT named hotspot.
This is all from the perspective of the day before EE WiFi was thrown into the mix. BT are renameing BT WiFi to EE WiFi now. First the network names have changed, then the website, and then the login page. This means that BTWi-fi is changing to EE WiFi, and BTWifi-X is changing to EE WiFi-X. I haven't found a way to connect to EE WiFi-X yet. I don't think BT/EE want it being used, at least not yet. It may be possible if you have the required technical skills to deconstruct a BTWifi-X configuration profile and fiddle around with it in an XML editor, But I think its unlikely. I haven't been able to auto connect to any of the renamed hotspots at all, and I totally understand your frustration. A short-term solution is to save your BT/EE ID password to your device and auto-fill each time. A more longer-term solution would be to try updating the BT WiFi app or downloading it if you don't have it already and trying your luck with the official auto-connect options. But note these only work on iOS and Android, not Windows or Mac. And I've never seen them work with any of the new EE WiFi hotspots.
