You mean, are there a few people who are just so likely to kill themselves, that they almost inevitably do so within a few hundred hours of getting their licences?
That is virtually guaranteed to be true, since only a tiny % of pilots get past a few hundred hours
It's like saying that virtually nobody succeeds in stealing the crown jewels before they reach 120 years of age
If you read American pilot forums (as I have been since Usenet used to be the main "forum" for aviation years ago) you find that the USA has the same problem as the UK in that nearly everybody chucks it in very quickly, and only the most determined hang on, for a bit longer, and then only the most obscessive types hang on after that
Over there they like to moan that the average age at Oshkosh gets 1 year higher every year and is now around 60
That said, Americans tend to hang in there a good bit longer than the Brits, probably simply because GA has a lot of utility value out there (H24 airports everywhere, good "going places" pilot training, etc, etc).
There is no "killing zone" once you adjust for hours flown etc. and that data doesn't publicly exist - in the USA or in the UK.
The UK CAA does have quite a lot of data which it for some reason doesn't publish (e.g. how long pilots retain their medical after the initial issue) - I believe because it would show GA in poor light but like the FAA they will have this kind of data so one could estimate the dropout rate fairly well.
What there is no decent data on however is annual flying hours. Informal surveys suggest that the UK average is somewhere in the 10-20 hours/year, but there will be a big standard deviation on that.