According to the response I got from Costa all their nutritional information is on their website.
It is very hard to navigate but the hot drinks that are vegan are:
Roasted Hazelnut Latte
Sugar Free Caramel Latte
Obviously these are only vegan if you choose the soya milk option (if there is milk). Hope this is helpful. Rather disappointing to me as my favourites that I thought were vegan - Chai Latte and Hot Chocolate aren't.
If you want a summary of the nutritional information that I have been emailed from Costa which is much easier to use, please let me know.