I can't speak for parents in the UK, but I gave my dd chocolate when she was younger than 1 year. I'd never heard that you shouldn't. I know you're not supposed to give peanut butter under two-maybe that's why no chocolate? Because you often see chocolate with that "Processed in a plant that processes peanuts" warning.
(Of course, I gave my dd a piece of peanut butter cup when she was like 8 months old because I wasn't thinking-but there's no history of peanut allergies or other food allergies in my family so it wasn't a big concern of mine anyway. But that's just me.)