They mostly do have the V label on vegan products.
However some just have it stated somewhere near the ingredients as "suitable for vegans" and then there's also some where they don't state anything but they are still made to a vegan recipe. But for the most part, at least for the kinds of things tourists will buy, you won't have to deal with the third one that much, if at all.
You'll be able to find loads of vegan restaurants in London via Happy Cow. It's been a few years since I've been down to London so I don't have any current suggestions. I do remember hearing there's a market (i think Camden) where there's a shop for vegan artisan cheese and that it's amazing.