I have got shetlands and also some commercial crosses, the Shetlands would go as 18 months old hogget (or older for proper mutton), commercials would be more the usual fat lamb stage at 40kg liveweight in late autumn/early winter. I am near St Boswells.
Would also depend how you wanted it - cuts or a whole/half for yourself to butcher? Problem now is out nearest abattoir is Shotts, so I need to do a minimum run of at least 6 to allow me to charge reasonable amounts. We now only do very few to sell as meat as not really financially viable. If you have your own transport/CPH I would prefer to sell live.