I have three levels of FAK. The one at home is a gert big cardboard box, containing enough dressings, drugs and kit to survive a minor zombie outbreak.
The second isn't really an FAK at all, it's a comfort kit that comes out with me when I'm on longer rides or at times when I'm in a position when someone ought to be carrying some kit. It's a small tupperware box, containing dressings, plasters, steristrips and the sort of thing that you'd need to cope with someone stacking a bike. It also contains Auntie Charlotte's magic briefcase - a selection of painkillers, stimulants and happy pills of various provenance which have proved themselves useful during the course of lengthy bike rides and aren't for consumption by anyone other than me or my cohort.
But on a day to day basis, I carry very little. Like many others, I reckon that the most useful thing is to have done some first aid training, to know CPR and to be able to manage the first few minutes of any incident. Knowledge is way more important than kit.
That said, this is what lives on or in my messenger bag, day in and day out:
Non-latex inspection gloves
Resuss mask
Benchmade ERT-1 Response Knife
Fenix LD01 torch
Mobile phone
My feeling is that in London, you're never more than a few minutes away from a paramedic. I don't routinely carry boo-boo kit on the basis that if it's minor enough to be treatable with a plaster, it can wait until I'm near a source of them.