In REAL LIFE the HQ's Main responsibility is the gathering, analyzing and distribution of information, and planning. So I believe the HQ being destroyed should effect the following:
1. Disables country channel. (local radio "room" channel and VOX still functions for players in range of each other)
2. country Dar Bar is disabled, but Local radar (in ring) still functions.
3. Automatic supply convoys are disabled.
It also should be hardened, requiring a minimum of 2 sets of Lancs scoring direct hits. and it's defenses increased.