You are assuming ONE plane, when we have been told that pilots were known to have trained in low level flying by following each other down a loch. If more than one plane traveled that path, there's no need to worry about the time between shots.
Sure, one has more time to maybe (re)compose for the next pass and indeed if a previous jet had passed then there is a good chance the next one may have a similar track relative to the available field of view as in the photographer had good visual awareness going on outside of the viewfinder so could time the shot.
A fairly high shutter speed would still be needed to freeze the jet's motion as we see it. It's possible to use a slower shutter speed and pan track with the direction of the moving object but then any other stationary objects in the shot have motion blur.
So to me, without pan tracking, the jet in this single frame is either a lucky shot or maybe pre-composed as suggested if it is not the first aircraft. If the camera was held steady or on a tripod whilst a number of frames were exposed then maybe this is the best (only) image of both, or maybe just got lucky with a single click shot. Distance to target is key.
If it's real of course...usual caveats apply.
I'm really curious to know what the focal length of the lens was - I know, I'm late to the party and there are over 46 pages on this thread so it may be out there somewhere in the comments but I've lost track so I'm probably repeating what's gone before.
Military jets on low level training tend to not strictly follow each other in line astern due to wake turbulence unless the horizontal spacing is large enough to be safe (at that time the RAF was cleared to 250 feet AGL and I'm sure you do not want to suffer wind sheer at that altitude).
(Formation in trail is a low aspect view of the lead aircraft and the more common practice is to offset to keep the lead in more of a profile view by flying in echelon formation).
How do I know this - I used to live under a low level training route in Wales (Brecon Beacons) and our cottage was in line with an entry point to the valley and mountains behind us. The timing separation was approx 5-30sec between aircraft flying into this particular valley as singletons, so if there was more than one aircraft at Calvine then for sure there is time to reframe in anticipation and plan for the next one. (In my experience, separation timings varied on the topography of the various low flying areas.)
My newly born daughter at the time did not appreciate being woken up by the jets! Nether did we and as a photographer I didn't have the slightest interest in photographing the bloody things! (Despite being into planes and wanting to be a pilot as a kid having been born and raised on RAF bases).
But, if anyone is interested in fast jet photography and techniques then this video gives a little sense of the famous mach loop in mid Wales. The stills shown in the video exhibit pan tracking.