I don't think this has been thought through at all, it would be an impossible task for smaller businesses, and high end restaurants to work out the exact calories in each portion. The small ones would go out of business. Jacket potatoes come in all sizes, and you going to have to weigh each one, and how would you put a generic weight on a menu?
The only sort of place it would work for is large chains like Wetherspoons, where it is all prepacked and reheated.
I sincerely hope common sense takes over. If you order a sticky toffee pudding, you know it is going to take you over the recommended daily allowance, that is your choice, and if you wish to eat it and live on soup and salad the next day that is also up to you.