COBRA

It doesn't affect the question, or indeed the answer, but according to Wikipedia there is no Briefing Room A and the A in COBRA doesn't stand for anything. The meeting rooms in the Cabinet Office at 70 Whitehall are known as the Cabinet Office Briefing Rooms (COBR); COBRA just sounds better, and having gained currency in speech it's been adopted in written communications.

Wikipedia spells out that there is a room in a different building that's labelled Conference Room A; but this is "not part of the COBR facility".