NATS has recently (last couple of years) changed to recording every infringement they are aware of. They then classify the results of these infringements depending on the impact on traffic in CAS. You can see a summary of this on the flyontrack web site. The classification are broadly
Minor - separation was maintained - 93%
A concern - standard separation was lost - 5%
Serious - The aircraft were quite close - 2%
NATS and the CAA acknowledge clearly that the trend in minor infringements is driven more by reporting than any change in activity.
non-transponder/Mode A only aircraft can only be detected infringing control zones (not areas) unless they full into the A Concern or Serious categories and hence they figure to a much higher proportion in the later two categories than the first one.
With regard to the comment
It is inevitable that pilots navigating visually will infringe airspace by small amounts
that shouldn't be true in general. However, for airports partially embedded in CAS or transiting some of the narrow gaps, the VFR flyer has to be able to navigate to the same precision as a two crew aircraft with PRNAV, and this clearly does result in minor infringements (which is why you see clusters around the London area GA airports).