Your terms are one thing, but what about the law? Genuine question, I'm curious! Wouldn't that be classed as interception under RIPA, i.e. a criminal offence? What about related legislation like the Wireless Telegraphy Act or the Computer Misuse Act? Surely intercepting communications between parties without a court order, whether they're sent plain text or not, is unlawful?
All this fuss with Phorm, and the ICO is still twitchy even though they've said the data collected is anonymous... I wonder what the reaction would be if they started reading emails?!
EDIT - Forgot to add, I'm with UK Online and they (Easynet/Sky/UK Online) have a massive fibre backhaul, plenty enough bandwidth for everyone. I download at 2.2MB/sec and transfer (up/down) well over 500GB a month. They said that's fine and they don't cap/throttle. Out of interest what ISP do you work for?