Pda, whilst the CoL down south may be higher there are many other advantages. Shorter distances, more shops, amenities, activities, more daylight in winter, warmer and so on. It is no question why people prefer the south coast to the empty north coast.
Back in the 70s there were several senior officers that refused Lossiemouth and promotion to gp capt. ISK was different as St Mawgan was the only alternative.
IIRC there was a similar tax issue back in the 70s. I don't remember if it came to fruition, but it depended if you were in quarters or private, and spouses working or some such.