There will be several historic reasons but, in the past, your property may have been part of MOD land such as former Married Quarters accommodation. Often, it is too costly to connect your property to the public networks, so the private supply arrangements remain for water and/or wastewater. You may wish to pursue the cost of connection to the public network(s) but your local water company might require you to pay a one-off connection charge which reflects the cost of making connections to public systems.