if the mains is low pressure, standard unvented won't work either you need 2 bar at 20lpm You could go OSO Dualstream it will work from 5lpm and provide hot water subject to a boiler or electrical heating.
This is an unvented system and must be installed by a competent person and registered with building control.
This is an unvented system and must be installed by a competent person and registered with building control.