So, possibly an experienced photographer. (Why not someone working as a press photographer at a tabloid—let's say the Daily Record…) But this leaves us with a problem: why is the photo so darn bad?
In my late teens (later 70s-early 80s) I was a rather experienced photographer. I used my father's equipment of course, I could not afford a camera myself, but we used a top 35mm camera, a Contax RTS, with a set of objectives and accessories. I took mostly macro colour pictures (diapositives), while my father also took many B&W photographs (using Ilford films, btw), which he developed in a small home darkroom (he also bought a 6x6 camera later).
One thing I learned soon is that I could expect to throw away more than half of the pictures taken, and get a really 'good' picture say every 4 or 5. Remember, there was no feedback at that time, one saw what the photo actually looked like after days. So with an interesting subject I always took more than two pictures, trying to change the parameters and hoping for the best (this was not always easy, some subjects stay rather steady, but butterflies and lizards were really hard).
So I don't think it's strange the picture is bad, even if the photographer was experienced. Actually, I'd say the picture being bad is expected (by chance or by design), because if it weren't there would probably be no mystery.
And if this one is bad, then all the shots taken at the same time would be just as bad, since this is supposedly the best one.
Very probably. Now, supposed there really were 6 pictures in total, the question for me is why we only have one (a copy, actually, and cropped, on top of that), and why the provenance is uncertain and impossible to verify. I take this as evidence that informations have been purposefully hidden/obfuscated in order to create a mystery where there was none (and in this case, the picture we have is the 'best' one in the sense of being the most mysterious and inexplicable...). I feel rather confident that if we had the remaining pictures (and maybe even if we just had the original uncropped shot) a reliable explanation could be worked out, and it'll be as discussed her on Metabunk: 'hoax', on a spectrum from fully staged (darkroom tricks, hanging models, figures on glass...) to 'lucky shot of a hillock reflected in a loch and something fuzzy in a corner'. By the way, I have reduced the probability of this last explanation (pure lucky shot) in my mind, because having the fence and the hillock +- in focus while the 'jet' is out of focus should be possible only if the 'jet' is nearer to the camera than the fence. I'm not 100% sure of this, it's hard to gauge what is in focus and what is not, but this effect would be easy to get with a hanging model or a sticker on a glass, but improbable to get randomly (the 'jet' should have been a bug flying near the camera at that exact momen, much less probable than, say, a rowing boat on the lake).
So at the moment my personal explanation is 'fully staged hoax' (both the UFO and the jet are models/darkroom tricks) or, with a somewhat lower probability, a 'semi-staged hoax' (the UFO is a hillock reflected in a lake, the jet is a model/darkroom trick). But I may be quite wrong on the focus argument, so take this with a great pinch of salt.