AFAIK, you don't need to opt in to the EE WiFi service.
When out and about just go to Settings-Connections-WiFi (or similar) on your phone, and look at the list of SSIDs. If there is EE WiFi available in that area, you'll see EE Wifi as one of the SSIDs. Click on that, and then you'll need to enter your BT ID and password.
Unfortunately, and unlike the discontinued BT WiFi, with EE WiFi you need to enter your BT ID and password every time you want to access EE WiFi.
