I was wondering how long it would take before someone latched on to this. It's a sensible idea. Even a simple dust mask could be a life saver. Being an infrequent tube user, I made my own simple kit up. I decided against festooning my Camelbak with cylumes for fear of being fingered by SO19. A kevlar suit is also difficult to compress in to a commuter-sized package. Mini-Maglite it is then.
The way things are today an S10 with all the trimmings and an NBC suit would be the most sensible option. I would add a hammer for breaking tube / bus windows and a good torch. Trouble is you would probably be arrested or shot for having such kit in your rucksack or briefcase.