Gatwick was a forced sale and sold for a minimum price of GB Pounds 1.51 billion, so I would guess that Stansted (also a forced sale) would go in the range of GBP 1-1.7 bn. Gatwick's new owners are selling off shares without floating the company.
Perhaps Magix would consider being a shareholder in Gatwick, Stansted or Glasgow, assuming that they follow the same model. Or buying shares in a publicly quoted airports company.