I am fairly certain this is merely a labelling oversight. I would consider it vegan and buy it (in fact I do buy the bar I think you are describing from time to time).
Many UK retailers do not bother to label their products as vegan simply because of a general lack of awareness about veganism until recent years. Major ones seem to be catching on little by little, but there are still countless "accidentally vegan" products out there. I find myself reading the ingredients and making a decision simply on that basis a lot of the time. Perhaps also worth noting that I have seen a few reports of products which clearly contained milk or honey being labelled as vegan, so I pretty much always read the ingredients anyway. More scrupulous vegans will look for a trusted trademark like the Vegan Society's, or ask the manufacturer or retailer for information.
Most people and organisations, including the UK Vegan Society, do not consider possible cross-contamination from dairy products or eggs which are not intentional ingredients to make a product non-vegan.
It is possible for sugar not to be vegan, but that does not seem to be the case for UK products, and would in theory preclude it from being labelled as vegetarian also (considering that alcoholic drinks processed with animal products are not allowed (at least by the UK Vegetarian Society) to be labelled vegetarian).
I would caution here that, at least at the time of my most recent visit, the Co-op were selling two Fairtrade branded dark chocolate bars, the 70% cocoa one with no non-vegan ingredients, and the 85% cocoa one including (for some strange reason) milk. Milk is clear in the ingredients list.