ICAO Annex 2 - Unless authorised by the appropriate authority VFR flights shall not be operated above FL200.
When setting FL195 as the base level for the European class C airspace the above ICAO requirment was taken into account.
Since FL200 is not a VFR cruising level, the highest available VFR cruising level is FL195 and that ties in nicely with the base of the class C.
However, it is recognised that certain flights (military flights and civil test flights, gliders! etc) will have to be accomodated within the airspace above FL195 and there are clearly laid down procedures in place for those flights.
When it comes to Class D airways, TMAs and CTAs, one must remember that VFR flights in such airspace are under an ATC service. Controller workload has a major impact on the airspace capacity. For IFR flights it is simple - everyone gets a slot time if required or if no slot must depart close to the planned time and the flow operates without any problems. VFR flights are even simpler - they have no slots, they have no requirement to depart close to the planned departure time and if the controller is too busy to accept the traffic there is always airspace below the airway where the flight can continue without restriction if it can't wait until the controller is less busy.
The last time I was involved in such things, military formation flights were no problem on the airway - provided that the elements remained within 1nm of the leader. However, the whole idea of being military and having an expensive military enroute ATC network is to provide flexibility and operational effectiveness - something that is not on the civil controller's objectives when it comes to military ops.
Military pilots are well aware that many of the standard UK entry/exit points (north point etc) cross busy civil airspace and often are only too happy to work military going direct.
In France, military flights are often filtered out on civil displays and one can be crossing a major civil flow under military service and the civil controller will not be aware that you are there.
Regards,
DFC