People should be able to eat what they want, but there is a real gap in the market for nutritious, cheap takeaway meals. It's so easy to make a tub of hot rice and vegetable curry, or soup, ready to eat, with minimal additives - it would cost far less than £2 to make, and I would so choose that over a cardboard flat brown thing in some rancid bread for £5.
McDonalds isn't even nice - people only think that because they remember Happy Meals. Kebab vans and fish and chip shops I can understand, but even so it's completely false that healthy food has to be lettuce, and nice food has to be un-nutritious.
I know there are already people selling soup / curry etc, but it's usually expensive, unhealthy, and less accessible than McDonalds.