The problem is, you're fighting a somewhat larger battle.
The nutrition information that gets the big billing is the stuff that has been decided to be the 'problem' stuff, so that's why you always get cals, fat, sugar and salt.
The law says this is the minimum information, so most food manufacturers and vendors don't feel the need to invest time, money and ink in investigating and printing the full information.
I had the exact same problem as has been highlighted here with Sainsbury's.
The problem is, you're not just challenging a 'silly' law. You're challenging a duff but well-defended scientific consensus. Carbohydrates are healthy, right 🙄? So why on earth would anyone need to know how many carbs are in their food? It's only fat, salt and sugar that are bad, right (and if it's sugar that was already in whatever it was, that means it's magical and doesn't matter!).
What you're challenging here is far bigger than just an issue of labelling, it's about getting the whole caboodle to realise carbohydrates are important and that there isn't actually an appreciable difference between sugar and other carbohydrates. Given Diabetes UK has a hazy understanding at best of the relationship between blood sugar and carbohydrate, they're not going to be a useful ally.
I would say McDonald's is actually one of the better labellers out there because a) EVERYTHING is listed on the tray liner - you will never see a more comprehensive nutritional listing for any other food place and b) their website is really good at incorporating this info too. Compare this with plenty of supermarkets that simply don't do it at - Sainsbury's in my experience are the worst offender for this. The situation is slightly better than it has been because AFAIK, you're still supposed to include a full breakdown of everything per 100g - it's just that previously, there was no requirement to put any information on the front of the packet, or for McDonald's, on the box.
But the reality is, you're really going to have to convince health authorities that sugar and other carbs are effectively the exact same thing, biochemically speaking. Good luck with that as the cereal manufacturers for one are going to fight that one tooth and nail. Public support is also going to be iffy. The public can get behind identifying whether something's vegetarian or not but given most people are so stupid that they blame diabetes on diet, the idea that McDonald's should put more information on their packets to assist people with diabetes is going to be laughed away with the 'eating in McDonald's is why you have diabetes' line.
