I pay my Class 2 NICs twice a year and everything else owed when I fill in my return. I like to get my done prompt too, just to get it out of the way.
The exemption certificate is indeed only if you earn less than £5,885 per year.
I don't know the exact details with regards to child benefit but as there is a cap on the earnings a household can have now with regards to this (£50,000+ in household money affects it I believe) then it could be that this is making a difference to you, depending on the overall incomings to your home.