With regards to the 5 BAS aircraft:
They spend at least 6 months of the year in (and ferrying to/from) Antarctica. In addition, some of them are sometimes used for shorter detachments to Greenland.
For the majority of the rest of the time they are in maintenance, nowadays usually in Canada, and I wouldn't class them as low-hour aircraft. In Antarctica they tend to fly as much as the weather will allow them to.
BAS operational planning can be 3 years + in advance and the 'flying season' is fairly intense, so the aircraft are pretty well tied up for BAS use only.
Equipment used for survey work, e.g. mag, camera, I believe is a NERC asset and not purely used by BAS.