I think you miss the point slightly.
How do you think your GETs get to the remote end?
They go off your PC as a TCP/IP Packet, routed via the backbone to its destination.
The backbone can mirror those packets without your consent or knowledge - do you see where I am going?
As BT are signed up to Phorm, and IF Phorm are found to be legal, I can see nothing really preventing them from writing a bit of code for the backbone (OK all the backbone routers) to mirror all http GET packets to the BT Phorm servers regardless of their (the IP Packets) origin. If AT&T in the states also sign up to the Phorm nonsense, it will be nearly impossible to stop this Pharming going on and you, the end user, will have no chance at all of opting out.
/me I'll get my own TFH thanks