The UK police investigates anything the government wants to signal it's dislike about. It's the UK governments goto version of putting you on notice they don't like you.
If there'd been serious chances of anything coming out of it, they'd have acted by now - they, and everyone else, knows the damage has already been done, in that going after The Guardian now won't stop coverage, and would in fact be pretty much the only thing they could do which would get Guardian plenty of support (out of self interest) from other media organizations that have so far given the NSA/GCHQ revelations very little attention.
If there'd been serious chances of anything coming out of it, they'd have acted by now - they, and everyone else, knows the damage has already been done, in that going after The Guardian now won't stop coverage, and would in fact be pretty much the only thing they could do which would get Guardian plenty of support (out of self interest) from other media organizations that have so far given the NSA/GCHQ revelations very little attention.