I suspect that this will fall to the local authorities to police as part of their licensing process
Restaurants and cafés etc already have to comply with allergy labels so in real terms I don't think that this will actually be such a big deal... The information is already on the packaging when it arrives from the wholesaler and I suspect that there will be a compliance time frame to enable the adjustment
Whether it needs to be done is a whole different question
Article I read says its about £500 for each dish which will need to be redone if ingredients changed. There will also have to be considerable latitude where all ingredients are different many by a considerable margin. Otherwise it could be a nice earner in fines for the government as big as motorist entrapment. What is will mean is that all meals are supplied by factories, like Brake Brothers, that can afford this, due to the volumes sold and then cooked in a microwave at your local restaurant, unless you are eating at a £100-200 per head restaurant. This will mainly affect adults as they tend to eat much more than children at restaurants, while this measure is being studied as part of reducing childhood obesity. The law of unintended consequences means that the only freshly cooked wholesome food will be there for the few that have the time and inclination to cook at home with fresh ingredients.
Even if they provide an opt-out for small restaurants like they have for 5p shopping bags it will only be a temporary measure, where the do gooders now want to make bags 10p and it to apply to all shops and our socialist government is looking to introduce this. Again the irony is that by far the biggest plastic polluters are Asian countries with much of that the dumping of Western plastic, where local authorities sell to the cheapest contract and the disposal companies illegally dump it.
What it does show is that the Conservatives are a party of big taxes, big government and big regulation socialism these days.
We need new right wing parties that aren't UKIP.