I'm absolutely positive they can do it. You could be right in the sense that maybe they're already doing it and this recent debate is just a Govt. play to drum up public support for an eventual law being passed.
We live in a world where people are persecuted for their race, gender, sexual preferences, wealth, mental acuity, religious beliefs and countless other reasons. Considering that, it's entirely feasible that someone, somewhere, could be persecuted for following a football team.
This essay says everything I would need to about the 'nothing to hide' argument and far more eloquently than I could put it:
https://www.schneier.com/essays/archives/2006/05/the_eternal_value_of.html
That's the key part of the debate. Who decides, and under what circumstances, the sorts of messages that can be viewed by the intelligence services? Messages sent by anyone with dark skin and a beard? Or all messages sent by former convicts? Or any messages sent by anyone who's been nicked for...