The UK's a bit small and the Fylingdales restricted area is even smaller, and embedded in the open FIR. Plenty of stuff goes around it at medium/low level and even over it at high level every day.
I suspect if QRA scrambled for every single non-communicating aircraft that got near the restricted area they'd be kept somewhat busy.
It's even rumoured that over the years more than one solo student, slightly uncertain of position may have got a bit closer than they should done....