Reminds me of the times we used to tap the 'Norwich Flyer' when I was on F4s! Setting up for medium level PIs in the usual area, the AirUK track in to Norwich was often similar to the target ac's track. It wasn't that easy to discern the difference on radar although you would have expected the navigator to have NB'd the target velocity being rather less than that of another F4! We always broke off when we became visual though - never joined close formation.
Quite a few ac off-airways used to get intercepted over the North Sea back then - many without ever having known about it!
Came back from Scotland after an exercise debrief once in the AirUK F27 via Leeds-Bradford (couldn't get in due to fog) and East Midlands (to offload the Leeds pax), then on to Norwich. Getting intercepted on the way by our own sqn F4s!!