Interesting. When I lived back home, my Dad had the usual boiler in the airing cupboard which was in the upstairs landing which never provided enough hot water. Over here, the method is to have a 40 gall. hot water heater usually located in the garage which is usually adequate. When we had to replace it, we considered an electric instant hot water heater but from our point of view, the cost factor was too high so we installed a 50 gall. heater instead at half the price. The roof structure here is similar to those described but roofs are much lower at the peak so impractical for much use beneath.
Brian.