Unless you have a BT business broadband deal, it's unlikely you'll have a fixed external IP address however you can sign up to a free Dynamic DNS services e.g. DTDNS, DYNDNS, NO-IP, 3322.org, dtdns.net (I use dtdns.net click here ) to overcome changing external IP addresses, allowing you to remotely access your IP camera.

1) Setup port forwarding on the router or the camera if the software allows this.

2) Signup for a free Dynamic DNS service to access your camera outside your network.

3) Setup a local static IP address( this is in the region of 192.168.x.x) for your camera by going in your bt HomeHub & choose using this IP address all the time in Advance settings.

Because if the local IP address of the camera changes due to router reboot or any other reason, the port forwarding would fail e.g. during the port forwarding setup the camera’s local IP address is 192.168.1.3 and port number 8xxx, and you try to access the camera from the outside your house, it will work. However, when the camera’s local IP address changes to 192.168.1.4 and you try to access the camera from outside the network on port number 8xxx, there router would not know where to forward the request to, as it doesn’t automatically update the new IP address of the camera.

click here for How to set up a home security system using Dynamic DNS