Just found this:
Defensive Air Suite
Mr. Lancaster: To ask the Secretary of State for Defence how many C2 aircraft in the Defensive Air Suite were (a) operational and (b) unserviceable on 31 January. [49117]
Mr. Ingram: On 31 January 2006 two RAF Tristar C2 passenger aircraft were available for operations. A third aircraft was undergoing scheduled maintenance. All three aircraft are fitted with a defensive aid suite and flight deck armour.