You may feel uncomfortable on a bike in London at first, I did. But at the same time that you are the most vulnerable road user if hit, you are also the quickest and most manoeuvrable, if you are aware of the dangers and keep your self out of risk where possible you'll be alright.
Precisely. I think new bikes should come with stickers that read 'This machine has no brain. Please use your own.'
I cycled in London for the entire time I lived there (12 yrs) and in that time I watched the average standard of riding really plummet, especially once the 2008 crisis hit. The roads seemed suddenly awash with clueless headphone-wearing suits on cheap hybrids, carving their way on and off pavements like a kid on a scooter.
The level of verbal and physical aggression between cyclists and drivers in London is definitely worse than other large cities such as NY or LA, in my experience.
I hold my hands up to giving some London drivers both barrels on a few occasions, but mostly those that should know better - professionals paid to use the roads (Cabbies, psycho Royal Mail Van drivers, etc). I was a pedestrian and motorist whilst I lived there too, so only gave it out when it was really necessary.
Cycling around LA is an altogether different proposition; 95% of motorists simply don't expect to see you there, and don't know how to react, not to mention that the speed limits on most streets are higher - typically 35mph which means most cars are doing 40+ when the traffic's good (there are no fixed speed cameras here). That speed difference puts cyclists on the back foot right away - you have to defend your position on the road but stay the hell out of the motorist's way, on roads that weren't ever designed with the cyclist in mind. The city is slowly evolving to accommodate the cyclist, but at this point in time it's like London was 12-15 years ago. On a 7-mile morning commute into Downtown LA, I see perhaps 5 or 6 other cyclists. It's a real change of scene.