I'd not waste my money on those diet plans / meals, just count calories and plan meals.
If you're relying on whatever the sandwich van has or the work canteen you're likely to make bad choices (you'll have no idea what you are eating so how can it be an informed choice?).
Just looking on the back of any food product you buy will help, be informed and if you still decide you're going to eat it fine, just know it. Planning meals (e.g. a pot of porridge you can make at work, a soup you can put in the micro etc) will help and if you can make a big batch of something on sunday then portion up you'll find it much easier to eat after work as you know it's there you won't be tempted to pop into the shop on the way back. You'll also know exactly what is in it, not only in terms of calories but in terms of grams of protein, carb, fat, salt etc. It's important to get enough protein and fat and eating the right amount of carbs will help your mood, sleep and exercise and make you a lot less likely to give up on the diet.
Also I don't like the term diet, it has the impression of a temporary thing and it isn't, it's just an awareness of how you are fueling yourself for the rest of your life. Hence why I think things like slimfast are pointless, if you can't do it for the rest of your life why bother for the next few months?