"Yet apparently a civil servant will have a significant part of his/her mortgage paid for several years should he/she choose to move from the sticks to the smoke and buy a new shoebox to live in"
Part waahed - there used to be a system whereby if you were moving location, then you could get some financial support to assist you for a limited period of time if you had to move house and family.
The issue with this was tapering of payments, so by all means get a mortgage £200K beyond your means, but be prepared to pay for it in a couple of years. The other means of support was to pay rent for a flat up to a certain ceiling, and all basic bills, but no food allowance.
Nowadays these moves have all but vanished - its incredibly rare to find anyone who gets a move package now, and almost all jobs require the new occupant to move location at their own cost with zero financial support.
So, if hypothetically I wanted to take a two year post in Bristol to broaden my horizons and skills, I'd be required to pay all the moving costs to get a new house in Bristol, and then all the moving costs 2 years later to move back. Alternatively I could spend 2 years commuting, with no support available, or I could rent a flat at my own cost and pay for it in addition o my own place. In other words, I can do what I want provided I pay for it. There are some limited exceptions to this, but they are rare and hard to come by.
Were the terms and conditions currently applied to CS extended to the military, then I predict a minor riot...