The point to remember about the British Army (Raised in all three Kingdoms) is that, before the 19th Century, it wasn't a unified body below the level of General Officer.
While a Brigadier and upward were part of the 'Army Establishment', the individual regiments (with the exception of Artillery, Engineers, Sappers, Waggoners etc. <1>) were in fact a form of franchise operation.

A would-be Colonel obtained a 'beating order' from the Government, allowing him to raise a Regiment 'by beat of drum'. From that point on, it was his responsibility to recruit, clothe, arm and equip his regiment (within the framework of the regulations) - he was paid a fixed fee per man for uniform, arms and equipment (on a pre-determined scale) and pay at fixed intervals all based on the nominal roll (and, yes, there were incidence of corruption). How a Colonel recruited was up to him and there are recorded cases of Highland 'Lairds' either forcably recruiting their own clansmen or indeed selling them to regiments due to serve abroad.

The point is, if the Highland chieftans in Charmion's posts simply handed over his men as a military force, then, provided they went of their own accord, they were mercenaries, however, IF the Chieftan had an order allowing him to raise a force for the Crown - albeit militia <2>(and in the light of the actions following the '15 and '45 - he'd be bloody stupid not to have) then the force is NOT mercenary, but legitimate forces of the Crown <3>.

Regards

W

<1> These were direct Government forces unter the Authority of the Board of Ordenance.
<2> Such as the "Argyll Militia" Raised by the Duke of Argyll as a Government Force (these, I believe were the Campbells of the Infamous Glencoe Massacre).
<3> A legitimate commission could mean the difference between exile and execution if caught on the wrong side of a dynastic struggle.