Three nut cases with knives = terrorist attack. You have sensibly hidden and when you pop back onto the street there is someone with a hole in them pouring claret on the street. Yes, you can improvise a bandage, but a shell dressing and quick clot is going to do more than £20 and running off to search for the nearest Boots...then trying to figure out where they have stocked their first aid stuff...or towels, or loads of cotton wool. Bummer, no one is at the till to take payment because there are guys with knives on the loose...
You are riding the London underground. Some nut on the train in front of you sets off a bomb and now you and everyone else is stuck in the dark awaiting rescue...or in need of walking to the nearest platform. A good light and maybe a dust mask, water and maybe a snack are going to do you more good than a £20 note...'cause you are stuck on a bloomin train in a tunnel 30m below ground.
Money goes a long way, and I don't dispute comments about training, or how people may react badly to traumatic sights, but there are times when having stuff on hand beats having money. Imagine if everyone carried a kit like that, vs everyone just having money.
Anyway, the OP may have said "worst case" but their first post was pretty clear what kind of situation they were considering, so can we please keep on track with that, and not wander off into personal versions of worst-case-society-break-down? MountainGoat, that means you, please.
Not sure about the heavy wound dressings. While they could be handy in the aftermath of an attack (assuming you have the knowledge and temperament to use them), the chances are that they will be carried around, unused, so long that the wrappers fall apart from old age!
I like Nigeltm's list pretty well. A friend of mine was also caught up in that 7/7/2005 bombing and had a very long walk from Brunswick Square home, south of the Thames. Lots of people stuck on tube trains then.