Caterers have to be registered with their local county council health and safety executive. They are also required to have third party and employee liability insurance, which you should probably check.
Anyone who comes to the BJC to make money would normally be expected to pay you for their pitch (usually a percentage of their takings). Monte has historically provided a fixed number of free crew meals.
A caterer may give you a better deal for being exclusive, or if you can show that other caterers won't be in competition. BJC 2010 had Monte selling good food at relatively high prices, as well as a school canteen selling extremely cheap and cheerful stuff, and this was apparently a commercial success.
If you do give a caterer exclusivity make sure you are confident they can supply the variety and volume of food necessary. Some jugglers also expect food to be available any time of day and night, whilst this isn't feasible the caterer may have to be open long hours.
BJCs have lots of vegetarians, who will be pleased if they can get some decent varied veggie food, although having ONLY veggie food will put other people off.
Running a crew tab with the caterer will make things much easier, since you really won't be wanting to fiddle with cash while in a rush. The BJC 2008 organising team had 15 plastic swipe cards that worked at the Doncaster Dome eateries. In 2009/2010 Montegriffo's Frying Circus catered (in 2009 he brought a friend with a BBQ to serve meat) and he provided a stock of tokens every day that could be exchanged for meals.
Also see Bar