Llamaman: OK new bases:
Pro's:
No MoD landlord to worry about.
No mixed airspace to squabble over.
Build what you like and never worry about a 2nd hand nissen hut with corrugated roofing and no heating.
Can move quickly if the situation changes.
Con's:
Can't IFR home (I follow roads). Can't grub beneath the cloud base and make landfall - CAA watching!
This means for those sites which don't have recovery aids, the cab could be out of position until the weather clears.
Might need their own security precautions - which is costly.
overall - civvy bases are best.