It appears that Captain Stable's biggest contribution to human wisdom seems to be in the Jet Blast forum.
But, anyway, on the topic: nearly all UK midairs have been at or below 1000ft. It is readily obvious that by the time one climbs through 2000ft there is much less traffic. At 3000ft there is almost none.
There are simply much bigger issues to worry about in flying.
The vast majority of traffic (reported by TCAS or a radar service) is never spotted; the Mk1 eyeball does not work very well at all. It's a fallacy, upheld by the Kremlin Old Guard since WW1.
And since a target on a genuine collision trajectory is a stationary point in your field of view (straight line trajectories assumed) you won't see him until too late.
The exception is when in the vicinity of an airfield, e.g. when landing. Then, TCAS is not very useful due to the possibly high # of alerts, and one has to look out as much as possible. Sometimes there is a real risk; e.g. if flying into Wellesbourne or Stapleford on a sunny Sunday preceeded by weeks of poor weather. Not a lot one can do about these cases - other than not fly to "free for all" airfields known for poor pilot behaviour when they are obviously going to be busy as hell.