I wonder how 'wild' the remaining UK mouflon are though genetically.
An archaeological centre in Peterborough have a flock of around 30 aninmals, apparently descended from just one pair. The closest sheep in the photo below is really a soay or a soay cross with possibly some mouflon genes, although the one in the background looks more representative:ther way, its not ideal to be mixing this species with any domestic sheep
Similarly, this Mouflon ram at Birmingham Nature centre appears to have some domestic genes in him:
whereas the sole survivor of a long inbred herd (the animal that prompted this thread) looks very different in appearance:
It seems apparent from these photos that the Peterborough animals aren't pure(or a least the ones pictured- the nearest one, as you said, resembles a Soay and the other isn't true Mouflon build or colouration either(though closer)
The Birmingham Ram in the photo has a white face and legs- obviously he's a cross with domestic sheep so its likely the others in the group are too).
The 'last Mouflon' however does look absolutely correct for the species so would say he is genuine. I hope they can pair him up successfully with another pure group(if any still exist).