https://en.wikipedia.org/wiki/List_of_English_football_transfers_summer_2015
Going by this only four players moved from Scotland to England last summer. Van Dijk who has been a success, Adam Matthews who has played for Sunderland twice and been punted out on loan, Erwin who has made no impact at Leeds and Botti Biabi who went into the Swansea youth team. Looking at the two transfer windows around it only Lewis MacLeod, Kayal, Billy McKay, Tony Andreu, Fillip Twardzik and Theo Robinson moved south and four of them were complete duds.
Scotland to England is not a thriving market at the moment.