"...Civil charter is vastly more expensive than the circa £43k per hour cost of a C-17....."
Surely that depends on a host of variables?
For example, I would presume (big mistake?) that flying an almost empty C-17 from UK to Cyprus (say 5 hours at £43k - call it £200k, although if it came back empty would that be £400k overall?) with a small cargo would be vastly more expensive than using a commercial carrier who is taking your load as merely part of a bigger cargo they are already committed to for other clients.
Conversely, charting an entire civil aircraft just for an MOD task might well be more expensive than the cost of using a C-17.
No doubt there is a very wide spectrum of circumstances between those two extremes?