The main culprit for possible incursions is the Russian`s. They seem to be fond of getting as close in as possible, and have been doing so since the late 1940`s. It`s all to test the other sides QRA reaction times, if any.
The problem is the Russian`s turn their transponders off, and don`t speak to ATC, so no-one is aware they are there, which has the potential, however remote, of a mid-air collision.
I did a quick " google " check to see if there was figures for UK airspace available, but most seem to be links to UK newspaper sites riddled with pop-ups, so I have not posted any links here.
This section of the RAF website explains the QRA for the UK....
Quick reaction alert | Royal Air Force (mod.uk)