It is not illegal because nobody has any image rights under UK law. If you have a legitimately obtained image, and you are not passing off, then the subject of the picture has no claim.
The Sex Pistols didn't get permission from the Queen to use her picture.
Passing of is very hard to prove. It is a common law 'tort', not statutory legislation, and it relies on three criteria - Goodwill, Misrepresentation and Damage to Goodwill. The plaintif has to prove that he has goodwill and prove that the defendant is misrepresenting the product as being real/official/genuine.
The law is also specific to the case in hand, and relates to the image and product in question. In the OP's case, Gilmour would have to prove that he had goodwill in that particular image ( or one very similar), and that there is likely to be confusion in the eye of the public.
In the Rihanna case I mentioned earlier, Top Shop had obtained an image almost identical to the official one, taken from a closed set. The garment it was used on was almost identical to the licenced one sold by River Island. The judgement was not an affirmation of 'image rights', but a judgement of that particular image and garment misrepresenting itself as genuine under the 'passing off' tort.