That sounds like a terrible way to drive. You'll be giving way to everyone, stopping for every pedestrian that wants to cross the road, slowing down at every junction.
I drive assuming everyone does what they are supposed to do and very assertively leaving little room for ambiguity. Driving in London for 10+ years, hasn't failed me yet.
You misunderstand what defensive means - imagine a rugby team or our armed services and their defences; you stand your ground or position. It doesn't mean allowing everyone to walk all over you or submission. As Rroff says you can be assertive as well as defensive.
I've driven in London many times, by far the worst driving I've ever seen was in Greenford during the morning rush hour - mega queue jumping, barging in, driving on pavements, ignoring traffic lights.
Another gem I remember - the only thing an indicator light indicates is that the bulb works.