Dave, use all your options.
Start by applying through the ballot. There's a substantial chance that you'll be rejected, but if you're one of the lucky ones whose name comes out of the hat you will have your own place and can choose your own charity, even a small local one that doesn't have golden bond places. You're not committed to raising a four-figure sum, so the fundraising pressure is off. Most big charities are happy for "own-place" runners to join their teams.
If you're rejected in the ballot, start contacting charities for a golden bond place and be prepared for some heavy duty can-rattling and jam-making.
If you haven't done a sponsored event before, it's surprising how many people are willing to donate. ou can assure them that if they are paying for you to suffer, they will get good value for money!
For future reference, the Great North Run and several other major events are first-come-first-served, so you can avoid the need to take a golden bond place if you're quick to apply.
Cheers, V-rap.