AFAIK you are not really supposed to take air systems on buses (bye laws) but if it's purged and hidden you should be OK.
It goes without saying that you should keep your marker well hidden in central London Just like anywhere else. Remember we have a lot of armed police around here.
Might be an idea to keep your mask in your bag as well, should keep nosy parkers at bay.
I'm getting the tube down with my kit and will be fairly careful as well.
I would be interested to see if anyone else has any further information regarding public transport.