I think you have to understand that Sky are not intentionally reducing your speed. There is no little man with pointed ears pressing buttons to keep your speed down. If your speed is being set and is subsequently falling then thre is a problem on your line which causes DLM to reduce the speed to keep the connection live.

You may very well get a better connection with BT, who seem to have a more amiable association with BTO than Sky. BTO will no doubt pull out all the stops to help their sister service, whereas they will do little to forward Sky's complaints. Good luck with your move.