Have a look at the Carbon Coop website (plus sister org People Powered Retrofit) - they've been thinking about this quite systematically for some time. There's also a lot of academic & think tank work on this dating back even to pre Green Deal. Off top of head maybe Green Alliance? Poss even Which?

PS DECC commissioned a fair bit of research back when they were DECC. Fact they then want on to ignore most of it is a bit depressing, but there we are....