You actually only need to be a UK resident without citizenship or a brit citizen living abroad also fine. I lived in UK for 13 years without being a citizen and signed lots of petitions for other matters without problems. As long as you put down a real address and post code it's cool.
To be honest the actual reasons are really not strong for this petition, it just sounds like a rant. Don't get me wrong I'll be happy if JW gets revoked their status, but what I'm saying the petitioner should have put better reasons who would interest also non-JW. There are many non-JW who dislike the WT but the 3 reasons written are just of interest to ex-JW who are possibly bitter to what happen to them, a non-JW when reading it would be like...couldn't care less, this organization isn't affecting me only those silly buggers who got involved with it.
Better reasons would be stuff like the organization doesn't actually do shit for the community like homeless people, or environment, if there's a natural disaster they don't have funds or plans that helps the community etc etc, those type reasons (explained in more details than what I wrote of course) would tickle more the interest of non-JW which lets be frank are the vast majority.