I've been following all threads on this subject from the beginning and would like to throw in my 2 cents, as I haven't seen anyone mention this hypothesis:
It's not an airplane, it's a bird that photobombed their Nessie hoax, it's flying between the camera and the top of the lake, so we're seeing it from top-down, flying from right to left. Then they went home to develop the film and noticed they had a better hoax than Nessie.
Any thoughts? What bird would fit the silhouette? The bird could had just took off or was coming for landing, thus it was adjusting its feet and flying close to the water, and the neck was not fully extended. Grebes are endemic to the UK and Scotland, but it may have been another bird if it's a bird at all and not pareidolia.
The lack of reflection from the "bird" is an argument against it being too close to the water, but still high enough that its reflection would fall in the cropped area of the photo, that would also require the bird to be closer to the photographer.
Another hypothesis that fits is that the "bird" was high up and the quality of the photo cannot tell us if we're looking at the dorso or the belly of the bird through its reflection. If it's the reflection of the belly, it explains why the webbed feet (or tail) are seen extended.