Anything is hard to find, if no-one ask for it! By which I mean that any shopkeeper is there to sell what she stocks. If they buy in a mutton carcase and then only sell two chops before they have to bin the rest, they won't do it again. In the UK, what's more you've rather lost the habit of breeding for meat mutton. From what I remember reading, you need a breed that gives tasty meat when mature, not meat that's too strong.
Actually, I bet that if you went to any halal butcher, you'd find it easily in fact. That's certainly where I used to get mine when I lived in Harlesden.