We've used a variety but my personal favourite is Lobbs near Bodmin for slaughter, Buttons at Michaelstow for butchery. You can arrange the whole thing through Buttons, and they will book the slots at and pick up the carcasses from Lobbs.
But if you're asking for this side of Christmas, I doubt anyone will be able to fit you in - we normally book all our November kill and cut dates in September, or even earlier....