I don't offer any kind of breakfast, but I'd love to hear from other hosts in London who do (I have a friend here who hosts and leaves guests a few simple things for their first morning only, but I don't know how common this is).
I do think it depends where you are located and also the kind of guests who visit. There are lots of places to grab breakfast a stone's throw from my house but also plenty of shops/delis/supermarkets, some less than a minute's walk away.
I give them instant coffee, regular tea and sugar, but also filter coffee (I have a coffee machine and caffetieres for their use) and a variety of herbal and fruit teas if they like. I tell them to help themselves to my milk. I often leave a bowl of fresh fruit in their rooms as well as drinking water.
Am I doing enough?
I think probably I am as most of them never even use the tea/coffee! No one so far has used the lovely display of different teas I prettily labelled. Although, having said that, a lot of them will take at least a couple of pieces of the fruit, so maybe I should continue with that.
What I do find is that, although I supply clean towels and travel sized toiletries, they seem to help themselves to other people's stuff in the bathrooms. Maybe those are more important to visitors to London!