Beafer, the data you have linked to shows only current assets and current liabilities.
The full accounts show that the airport still has net total assets.
Admittedly in the last year reported, a big lump of value has disappeared out of the company's balance sheet, which is not reflected in the trading figures.
If you want to know why, the answer may be in the full accounts deposited at Companies House, which will cost you £1.