Why not fly perfectly ordinary planes into a neighbouring country and then use military transports for the hop into Afghanistan ?
Lets think about this.
1. Name a neighbouring country which is going to grant diplomatic clearance for this longterm activity without making itself become a target?
2. An operation like this would further drain the limited resources of movers having to be based in a neighbouring country to ensure the smooth flow of pax between the military transport & perfectly ordinary planes, whatever they are.
Personally, I would not want to get on one plane in UK, unload and stand around in a neighbouring country, waiting for my next lift to arrive before re-boarding and continuing my journey.