If we take on face value that a border with red tape etc would reduce our exports to England by 15% then the opposite is also true and imports from England would be similarly hit. Given that England exports more to us than vice versa that would leave a big gap in our markets that Scottish companies could fill.
You are correct that a cheaper currency would boost our exports, not just to England but worldwide.
Its too simplistic to look at currency appreciation as good and currency depreciation as bad