OAT didn't make £18.5 Million last year. That was their incoming sales revenue (turnover). Only a portion of that would have been profit and probably a small portion considering the investments they have made in infrastructure over the past few years. Looks like ploughing the revenue back in paid off for them.
GECAT did not buy the airfield, though you can bet BBA PLC will sell it when the price is right. It looks like they are using the same strategy there: invest (new hangars, wider runway, ILS), improve revenue, sell on when they can.