But you couldn't do that for less than £3, or at least you'd struggle to, so it's all relative.
It's certainly 10x healthier than KFC/McD/BK, and no worse (if not better) than a pre-made supermarket sandwich which millions of us buy every week.
My daily lunch costs me around £3

. Has around 800 calories, lots of good fats, protein, fibre and vegetables.
I bulk buy my meat from an online butcher or butcher and freeze it. It's a big expense in one go, but depreciate it over a month it works out to be at least 10-15% cheaper than a weekly shop.
I bulk buy extra virgin olive oil from Makro or equivalent. This lasts me 6 months at least.
Bulgar wheat/quinoa/cous cous lasts a month at least so the cost is spread over a that month, and bought by the kilo so that's 3kg spread over several months - again saving a lot of cost after a large initial outlay.
Getting lunch around Canary Wharf in London which has anywhere near the nutritional profile of my own lunch would cost well in excess of £5
Typical lunch - with daily break down of cost (approximate):
250g chicken breast - £1.25
bacon Lardons - £0.25
Mixed salad / spinach - £0.20
Cherry tomatoes - £0.16
Cucumber - £0.15
Bulgar wheat/quinoa/cous cous - £0.10
Mixed seeds - £0.12
Pine nuts - £0.30
Olive oil - £0.10
Takes me less than 10 minutes every morning to prepare - and since I'm cooking my breakfast anyway it makes no odds.
I save in excess of £100 a month cooking my own food for lunch, and with the level of knowledge that I have anyway in nutrition it would be daft of me not to make my own food anyway.
People are lazy. People also don't really understand the poor quality of the food that is put into something like a subway sandwich or premade sandwich. Most of what you pay for are the overheads and the wages - the food products contribute to a very little amount of cost.