A thought that occurred to me as I was writing – perhaps they aren’t interested in the contents of the emails as much as who is talking to who – i.e. who should we (the security forces) be looking at next. Which (to me) makes a lot more sense that archiving peoples “lolcat” emails to their mates. The security services already have a watch list so legally being able to see who is sending and receiving emails from people on your watch list would be a good way to find other potential terrorists.
Not that a Network Security bod like myself knows the methods used by the security forces….