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I'm no expert (yet!) but I have spotted some foods in your list that are high carbohydrate. These carbohydrates will turn to sugar in your body. Biscuits and sweets you've identified as bad yourself. But other baddies are cereals, so your granola isn't good for you, oranges and bananas are high in fruit sugar, berries are better. I'm not sure about SW meals, they may be low in calories, but do they include rice, pasta or potatos, as they are all high carbohydrate. Your fast food lunch may not be good either depending what it is?
On my work days I box up a salad, chop up different salad ingredients into bite size pieces then add, chicken, ham or salmon, you could prepare this the night before. Nuts are a good low carb snack, you could try berries as I've already mentioned or pork scratchings for something savoury. M&S do something called Crackle crisps, they are basically fresh flat pork scratchings, they are in the fridge. A bit pricey but good for an occasional treat.
Another thing, do you have a glucose meter to self test your levels, I recommend you get one so you can see what different foods are doing to your blood sugars.

If you haven't already got a meter, you really need to buy one. Using a meter to test before and after you eat, and keeping a food diary, will show you at a glance what that meal has done to your levels. More than 2mmol/ at the 2 hour mark and there are too many carbs in that meal. This gives you the opportunity to tweak your meals, eliminate certain foods and reduce the portion sizes of others. This is called eating to your meter and is the best way to control this disease.

Looking at your typical foods, I know for certain that granola, banana and oranges, plus the biscuits etc would raise my levels a lot. They are full of sugar. All carbs turn to sugar once inside the system. The more carbs, the more sugar. Have a look at the nutrition labels on the packets and tubs of your foods. Look for the total carbohydrate amount per 100g. Then decide how much you are going to eat and work out how many carbs you will be eating. Try to keep this under 10g per 100g. Less is better.

The main culprits are bread, rice, potatoes, pasta, breakfast cereals and most fruits (especially fruit juice).

Slimming World is a calorie controlled way of eating. Most meals are full of carbs and not suitable for diabetics unless you miss out the carb content.

A quick grab and go may be difficult. It is sometimes better to take your own food, which you could prepare the night before as you get up so early. Any cold meats, tinned fish, cheese, eggs, tomatoes are all good. Non-coated chicken from a hot food counter in a supermarket is good.

I would just like to add that the low fat meals as recommended by Slimming World and Weight Watchers sometimes contain more carbs than regular meals in the chilled section of supermarkets, the same goes for artificial sweeteners. If you are making home cooked food from SW recipes then do a bit of research on the nutritional information of the ingredients. Avoid anything made of white flour, do not be swayed by the term 'wholegrain' either, that just means some of the grain hasn't had all of the outer coating removed and the rest is just white flour. I have not come across a bisciut yet that is low carb and sweetener free.
As for diet pop, observational studies have been said to show no significant weight loss and in a couple of studies there was said to be a small weight gain. The most likely cause (no definitive causal study has been done) is that the body expects a high glucose intake so increases insulin output then when the glucose is not forthcoming the body sends a hunger signal out and you go looking for a snack ergo weight gain.
Artificial sweeters are now being linked to problems in the gut biome.