Last time I checked, Lindt 70% and 85% were vegan. But that hasn't been for a while because I don't like Lindt very much.
Côte D'Or dark chocolate is vegan, their 86% isn't bad.
If you want fair trade or organic, as mentioned Divine dark chocolate bars are vegan and have a really nice texture, but they only go up to 70% and I need it darker to get my chocolate fix
Montezuma's dark chocolate bars are vegan I believe, but I've never bought them and I've only ever seen them in Waitrose.
Dark Rittersport is also vegan, as is their marzipan but apparently that's no longer to be found in the UK. Peppermint used to be vegan but butter oil has managed to make a guest appearance now. Anyway in the UK, I've only ever seen Rittersport chocolates in Waitrose.
And yes Asda's 85% cooking chocolate is surprisingly good, though it was a hunt to find it in the baking ingredients section. I hate Asda layouts.
Waitrose also stock the Lyme Regis marzipan, coconut and hazelnut nougat dark chocolate bars, as well as the Humdinger "milk" chocolate rice crackle bar and "milk" chocolate buttons which have all been mentioned before.
That's all I can think of off the top of my head... I've never had trouble finding vegan dark chocolate to be honest. At the moment I have the Côte D'Or 86% bar at home, but I've not needed to tap into since early October as I got given heaps of Hotel Chocolat chocolate
People know me too well!