Undertaking / filtering on left?

Yes, of course you need the skill to make the right judgement call each time. It's not that hard to anticipate what a queue will do, though. If you watch the traffic signals, which should be visible ahead, you can predict when you can slot yourself into the queue again. There really shouldn't be any 'suddenly' if you pay attention.

I generally agree with your sentiment, but I'm having a slightly hard time resolving that with my experience. I ride a fair amount and try hard to be aware of what's happening in order to not get caught out. Traffic is fairly predictable at lights, but coming up to T junctions, roundabouts (including my favourite binary mini-roundabout in Fulham/West Ken) things change more quickly than manoeuvres can be executed.

The right thing to do is to play it safe of course, but I think the safety margins are wide enough for a little ducking and diving.