How have I not eaten enough today?

Nowhere near enough calories, you don't need to stuff your face with chocolate to reach 1200 either, 1200 is actually a very low amount and you could easily make up your intake by increasing your portion sizes and not regarding crisps as lunch, that is not a lunch, it is a snack.

You are never going to get adequate nutrition from the example you have posted here and being malnourished is going to do nothing but make you seriously unwell anyway.

I used an app like that and it quoted me 1200, keep in mind I am in my 30's, short and sedentary, 1200 is not suitable for most people and should be discussed with a dietician first so that they can advise you on how to get decent nutrients from those calories if they agree that you actually need to go that low.

Please be careful, I went on to develop anorexia nervosa and although that might not happen to you the results of undereating and being malnourished are the same, the body does not cope with it and you become really poorly in the end.

I am type 2 diabetic and have to stick to healthy eating and have lost 28lbs, I have made a few changes to your list
Breakfast -ok
but I would watch the juice as it has empty calories of sugar

Lunch
2 slices wholemeal bread with chicken and salad or baked potato with beans

Dinner
- where is the protein from red meat or chicken?

Snacks
- Small banana : 90 cal
- Small handul of peanuts : 35 cal
-Helter Skelter Ice lolly : 58 cal - full of unnecessary calories and sugar, have another healthy sandwich or home mead soup

You're pretty good on vitamin fruits and vegetables. What's missing in your daily diet is high-quality protein and enough carbohydrates for energy. A hard-boiled egg or lean meat plus small servings of potatoes or bread would be good. Also you could have fifty calories or so of butter or oil. Just avoid sugary stuff – once you eat any, it sets up cravings for more and more of it.

Eating more than an ounce or so of chocolate can give you messy digestion.

Try eating a proper meal instead of cucumber slices...