Excellent point. I wonder of that is WHY somebody decided it must be a Harrier, in spite of the very poor image quality making unclear to me if it is even really a jet
The original claimant(s) said it was a jet, which is why many people interpret it as such- if it's a hoax then that was the hoaxer's intention.
Though we don't know how much
real effort* was put in to examining the picture, the UK Ministry of Defence identified it as "probably" (IIRC) a Harrier , and (frustratingly for us) identified a second object as also probably a Harrier in one of the pictures; frustratingly that picture (and the relevant detail in that picture) isn't available for us to study. If it showed greater detail, maybe "our" supposed Harrier was also clearer (but this is supposition).
We should remember the MoD also said no Harriers were flying in Scotland that day.
And- and I realise this won't go down too well here!-


-to me it looks like a blurry picture of a Harrier. The general proportions and layout, the (possible) sweep/ angles of the (possible) wings and tailfin, the slightly broader (possible) fuselage fore of the wings, (possible) relatively slender tapered nose. My
impression is that the putative wings roughly resemble those of a 1st generation Harrier (GR1/ GR3, AV-8A/C, Sea Harrier) more than those of a Harrier II (AV-8B, GR5 onward) which have broader, less-swept wings of greater area. The nose suggests (to me) a GR 3.
If the "aircraft" image is a less accurate portrayal of the actual object than I
feel it is, I might be persuaded it's a Hawker Hunter; the broader (possible) fuselage in front of the (possible) wings indicating a side-by-side trainer, small numbers of which remained in service (not least at Lossiemouth, Scotland).
Of course, the image is poor, and all the above para. can be effectively called into doubt by anyone else saying "It doesn't look like a Harrier, or a jet plane, to me" as my impressions are subjective, and the image insufficiently clear to allow e.g. measuring of relative proportions or angles between "wings" and "fuselage"; I can't support my impression with objective facts.
I could be accused of saying "I think
I know what I see in the blurry 3rd-hand image of doubtful provenance", and I'm not unaware of the irony of this (and the shaky basis for my interpretation).
I strongly suspect the overall picture is a hoax; if so, the "jet(s)" have a number of explanations, which might be:
(
A) A model jet used in composing the image. I feel Wim van Utrecht's examples show this is
relatively easy to do.
(
B) Something, not an aircraft or model of one, that was present when the photograph was taken but which was described as a jet by the hoaxer(s) and which resembles a jet in that (false) context.
If the "UFO" is a reflection of e.g. a rock and the "jet" a rowing boat, this would suggest a small islet. If the picture was taken in the vicinity of Calvine it's improbable that no-one recognises it, Perth and Kinross is not Siberia. A reflection of a bird, or a piece of floating debris in a smaller-scale setting cannot be ruled out. If there were 2 jet-like details present in one of the photos, the probabilities of two birds or twigs resembling jets, described as being "probably" of the same type, might seem lower.
(
C) A real jet fortuitously photographed at the time that the "UFO" was photographed. If the UFO is actually an object and its reflection, and the photograph is viewed upside down so the darker half of the diamond is topmost, the jet is flying inverted.
(
D) A photographic detail of a real or model jet, or other image meant to represent a jet, incorporated into the main image
(
E) A photograph of a scene including a real or model jet, or containing some other detail(s) that can be passed off as a jet(s), into which the UFO has been incorporated.
The photographic analysis by David Clarke's friend at Sheffield Hallam University
might be taken as ruling out (D) and (E).
(I'm not 100% sure that we've conclusively ruled out the possibility that the picture might in some sense be real.)
_______________________________________________________________________________________________________________________________________
* Possible scenarios outlined here
https://www.metabunk.org/threads/debunking-humor.132/post-336434 
.