All take off and landing zones have to be fairly wide for the planes to avoid wake turbulence i.e. the disturbed air that comes off a plane as it passes through the air. Follow closely behind another plane and it is a very unpleasant experience which is why the red corridor is so wide.
I'm not sure this is correct.
The P-RNAV specification that is planned for arrivals into London City is +/-1 nautical mile horizonatally, for 95% of arrivals, in practice these will be flown much closer to the horizontal track than this.
Wake turbulence is not limiting in this - the minimum permitted spacing varies between 2 and 4 mins depending on the aircraft type.
Driving in on the M40 on a Sunday night you will see a procession of jets on the same approach at these spacings.