From the Food Standards Agency:
http://www.food.gov.u...essential/startingup/
Remember that anyone preparing food has to have at least a basic food hygiene certificate.
If you'll be selling at any location where your vehicle, or a person purchasing food, will be standing on any part of a public highway (which includes laybys, verges, other roadside locations and footpaths), you'll need a street trader's licence from the local authority. (Many authorities often refuse to grant such licences to burger vans).
Many burger van owners who have regular trading sites now prefer to rent a space on private property (which might come with the bonus of a power supply). However that might lead to a requirement for planning permission for a 'change of use' for the land on which the vehicle is regularly parked.
While there's no statutory requirement to have any form of insurance (against claims arising from customers who claim that you've poisoned them!), anyone who arranges festivals (etc) is likely to insist upon you having such insurance before they'll sell you the right to trade at their event.
I used to do the wages & PAYE returns for a guy who employed several staff on his catering van. He clearly made a decent living from it. However I currently know a lady who has taken out a loan to buy a burger van, and who is paying for the site rental (including mains power). She nominally trades from 7am to 7pm, 7 days per week, although she'll go home early if she hasn't seen any customers for ages. While she says that she might just about make a living from it once she's paid off the loan, she's currently only making about £30 per week (for all of those hours!) after paying all of her outgoings.
Chris