That’s simply not true.
Small caterers are quite right in saying that this would be very difficult to manage. Which is precisely why they won’t have to (although large chains with the resources to put nutritional information on their menus are in many cases already doing it).
However... caterers can choose to give the information voluntarily, in which case there is guidance on how to present it. For the vast majority of caterers this will be an unattractive prospect, but for those who have moved into more specialised markets, including free-from, this might be another niche to explore. It's worth remembering that various surveys have shown that 1 in 4 of the adult population is on some kind of "diet" at any given time, either for weight loss or health reasons, and as the anti-obesity campaigns continue this information could be a selling point for some outlets.
The government guidance, with hyperlinks to further info, is here: