Most of the useful information about organising a British Juggling Convention is in the heads of people who have already organised one. If you're thinking of running one yourself, you would do well to contact any of the previous organisers, particularly those who have added themselves to the Who can help? page.
There are also some very useful online resources.
BJC 2016 have produced a general BJC website at http://www.britishjugglingconvention.co.uk
The JugConOrg Yahoo Group. Lots of organisers of previous conventions read this list: if you need help or have questions about running a convention, this is a good place to ask.
Nicole Taylor wrote an article on "A guide to running national juggling conventions" and Jon Relf wrote an article on "What do you do at a juggling convention?" both could be found on jugglingdb.
There is also the legendary box o shite, a collection of paperwork from previous BJCs.