So one of the things that you have to contend with in London, as a commuter on the roads, is the high number of traffic lights compared with the distance travelled. My route in to the office is 15km and mostly on one main road. In that 15km there are 75, yes, seventy-five, sets of traffic lights. Some are in clusters of two, three or even four within a few metres of each other. e.g. a cross roads with lights, followed by a set of pedestrian lights and then maybe a T-junction all within 100m section of road.
So, on my pretty typical ride home, 24 sets of those 75 were at a red signal when I got to them, resulting in almost exactly 9 minutes of being completely at a stop vs 39 minutes on the move (including speeding up away from lights and slowing down to them) in a 48 minute total journey time. Now, that was a slow trip home for a combination of reasons; tiredness, weak legs, gusty wind and some reasonable traffic. But that's an awful lot of actual stopped time, plus a lot of related slowing down and speeding up. I can see why some people jump red lights. If I could save at the very least 9 minutes off my journey, that'd make a huge difference (and that's not even taking into account the loss of speed to get to a stop and the ramping back up to speed again).
But. The law, and common sense in very many cases, says stopping at red lights is what you do.
For comparison, the ride up to Hertfordshire the other week took 03:20 to cover 76km vs. 00:47 to cover 15km. So, if I'd hit the same average speed for the long ride as for my commute, it would have taken 03:58 to get up to Herts. Or, if I'd managed the same average speed for my commute as I did on the long ride, I'd take 39 minutes to get home. (looking at my ride in, I've managed that in 38 minutes once, including 6 or so minutes stopped at lights... the ride home is never that quick)