I'm sure that it wouldn't have been too difficult for the government, had they really been worried about security issues at BHX when the Afghan refugee / repatriation flights were operating, to simply and at short notice designate BHX property as a restricted area under the terms of the Official Secrets Act and ban people other than those on legitimate business (passengers and employees) from the airport for the duration of the operation.
I don't know if anyone has actually approached the airport management for an explanation, though if they did I doubt they'd get anything remotely sensible from them; just the usual platitudes and fob offs that you get these days from corporate bodies.