Further, who exactly do you want charity status revoking?
The fact of the matter is 'Jehovahs Witnesses' do not have charitable status as per the petition question. I'm assuming you're refering to the Watchtower Bible and Tract Society of Britain, who according to the Charity Commission, its aims and objectives are to advance the Jehovahs Witness religion. They print, publish and organise the religion itself, so it would be difficult to prove the public benefit (or lack of) argument, due to the fact that they can distance themselves.
Further, every single congregation in the UK is a separate charity registered in the Charity Commissions register. So when refering to the term Jehovahs Witnesses to have their charitable status removed, you have to make clear who exactly you're talking about with this petition question.
The whole petition question is flawed, from the wrong legislation applied, to the wrong wording of who you want removing, as 'Jehovahs Witnesses' do not have charitable status in the UK. It would be thrown out in a Parliamentary debate on this basis alone, before any real discussion took place.