The low carb and low calorie (800 calories a day) diets are quite different and carbohydrate requirements/restrictions will apply to the first and not the second, so you'll need to be clear which one you're actually following. If it's low carb then you're looking at limiting your carbohydrate intake not calories, and you'd be much better off eating low carb (above ground) vegetables rather than higher carb porridge. But if you're restricting calories then that's the only limit you need to stick to. You should obviously keep to a healthy variety of food, but porridge wouldn't necessarily be an issue. I believe there are menu options for this diet, so you'd be best looking at these for guidance.
Robbity