A few more suggestions:

Around the UK, Hotel Chocolat. Some locations have a drinks bar serving their various flavours of chocolate drink.

Not hot chocolate per se, but I am a big fan as I don't actually like the thick, sweet beverage discussed here. Dishoom (Upper St Martin's Lane, London) serves chocolate chai. It's hot, not too sweet, and spicy. It's one of my favourite places anyways in terms of location, food, athmosphere, and servers always eager to chat.