Urban survival kit.

Now an urban survival kit isn't the same sort of thing as a normal survival kit, it's not intended to keep you alive in the wilds of the Welsh coast... It's more designed to make life easier (and maybe safer) in the wilds of London.
1.  Bottle of water.  Handy if your stranded and it's hot.
2.  Multi-tool. For all those little jobs that turn up.
3.  Electricians neon screwdriver.  The one that lights up to tell if the power is on, handy for changing a plug etc.
4.  A few plug fuses.
5.  Insulating tape or duck tape, for repairing pretty much anything.
6.  Chemical light sticks.  The ones you bend to make them light.  Get ones that will give a good light for several hours. Handy if your stuck in a power cut or the tube breaks down (it does happen)
7.  Dust mask. Get one thats designed for fine dust, this is if for either building collapsing dust or possible chemical attack
8.  Safety goggles (see above)
8.  A small first aid kit, just a few plasters, anti-septic wipes and a small dressing and surgical gloves (non-latex).
10. Shoelace, a spare for your shoes or boots.
11. Spare battery for your mobile phone, these are cheap off places like Amazon and Ebay. You could be stranded and need to keep in touch.
12. Cash, keep some real money in case the power goes down, a credit card is great but if the power goes out or the the card system dies your screwed without cash.
Finally... A waterproof poncho, one of those thin cheap ones you get at concerts/events to keep you dry if you get caught out in the rain

Now this stuff will pretty much vanish if you have a backpack or laptop bag, and it will almost certainly come in handy at some pont... even if it's just for changing a fuse!