I think the description 'feral' may not fully apply to sheep such as North Ronaldsays and the Hirta Soay, as they are managed to some extent. Ronnies are allowed onto the good grass for the summer months with their lambs, and Hirta Soays are closely observed by researchers and tagged. They used to be, and maybe still are, fed titbits by the forces outpost on St Kilda. So partly unmanaged.
I don't know where other feral flocks are, although I've heard of the Ronnies on Linga Holme.
One of the problems with feral flocks, is that visitors object mightily to seeing sheep dead or dying from lack of shepherding - a question with its pros and cons.
There are other flocks of feral sheep on some of the Scandinavian islands, and I'm sure elsewhere too.
An interesting topic.
Another aspect which interests me, is how much do previously feral sheep change when they become managed? For example, Soay tups are notoriously belligerent towards other tups, as they should be because of the way they organise themselves in their native home. However, having only a single tup for a flock of domesticated Soay ewes, means that the tup has been chosen by humans, probably using different criteria than the ewes would use, or that survival of the fittest might supply.
How hardy do these sheep remain once they have lived under human supervision for 100 generations? How are mothering abilities affected by birthing ewes being assisted if they have problems?
How and how quickly do previously domestic sheep revert to being feral if they are, say, abandoned on a remote island. I believe this has happened off Australia, and white sheep gradually were overtaken by coloured sheep, and grew smaller.
Lots of other questions out there.