Crash categories are used at every airfield. In its purest form it is a highly complicated formula involving fuel loads, weapons, materials, aircraft size, etc, to decide upon the minimum ammount of firefighters and vehicles required to save life should an incident occur. This is known as the critical area concept.
As i have stated on previous posts the fire service only has 3 minutes to save your life, if you become involved in an incident. Of this we are allowed 2 minutes travelling time, and only 1 minute to make conditions survivable. To do this we need to effectively deploy adequate (and properly manned) fire vehicles, and be capable of discharging a sufficient ammount of foam and water within one minute, with a continual requirement for the following minutes, until the situation is concluded.
I'm trying to make this simple, but hope it still makes sense.
Coningsby operates at crash cat 3. The Hercules is a crash cat 4 aircraft. Therefore it cant land at conningsby without the crash cat being upgraded.
However before you all start jumping down my throat blaming firefighters for being lazy, please spare a thought for the fact that air traffic would be authorised to accept a below cat aircraft up to a set number of times a calender month, (locally set by a different calculation method) and ultimately the decision rests with the captain of the aircraft. He can land at any airfield at his discretion, regardless of crash cat.
Whilst I dont know anything about this specific case, I would be tempted to beleive that the crash cat is being used as an excuse.