The military don't get the benefit of "free" housing. They pay rent like everyone else. A rent is paid to some formula based on the average paid for civilian owned property of similar standing. This is so personnel neither gain nor lose out because of the vagaries of the military posting system and different rents charged in different areas.
Don't think this is quite true - you pay effectively a wear and tear tarrif and a charge in lieu of council tax. Before I left ( last year ) I paid £120 per month for a 4 bed detached house in Hampshire outside the wire. I'd love to find a similar property on the civ market. The council tax equivalent was £150 per month.