Banksy - Top current London locations (artofthestate.co.uk)
BLUE PIN - all still there but probably under perspex maybe with some minor tagging
RED CROSS - still visible but not in original state, some major detail of the piece will have gone or been obscured
RED EXCLAMATION gone.
The best historical guide to Banksy's London locations is Martin Bull's 'Banksy Locations and Tours' book but if you've only got limited time and want to be fairly sure that the graffiti will be there when you get there try these locations. If you visit any of these locations and find the graffiti has been removed let me know (and I'll quickly take it off the map.
Banksy's work is now pretty spread out but London also has a wealth of street art from other artists. If you're interested in a tour there are plenty on offer but to see the best bits I'd recommend choosing www.shoreditchstreetarttours.co.uk. The guy who runs these is friendly, very knowledgeable and always on top of whats new and interesting