I'll be traveling to London soon and would like to have a look at some professional street art, such as the graffiti works by Banksy. Is there a continuously updated map somewhere that lists the location of such works?
While it's not a map, why don't you consider a tour such as this free walking tour (no affiliation, a quick google search will give more results).
I've taken a similar tour in NYC and was quite pleased. As a bonus over a map you get some background info on artists and tools, typically the guide is an artist that can point out more locations to you.
Best I can think of is Legal Walls, which maps graffiti locations around the world.
While you are in London, be sure to check out the Leake Street Tunnel [ images here ] - a three minute walk away from the London Eye, which was set up by Banksy as part of the Cans Festival in 2008. The tunnel used to form access for the now defunct Eurostar terminal at Waterloo station.
As I used to work on that street, I know that the artwork can last a little as 24 hours before the next artist sprays their work all over it.
A website called "Street Art with Google Art Project" offers a great online map of graffiti around the world, including London:
The only problem is that the map is not too detailed, so it would take some walking around to find the actual location of each work.