It is WAY more expensive to eat out, no matter how cheap the fast food. You can make a home cooked meal for 30p per person and most fast food places charge a pound for chips and a few pounds for anything with meat in it. For the cost of a burger, chips and drink at Burger King you could probably cook the same food for six people at home.
It's about 50p for a kg of potatoes, 11p per burger bun, £3 per kg of mince (Tesco) so you could feed 6 for under £5.