You seem to have a commercial interest in selling / servicing Solar Thermal - a point you neglected to reveal in your last comment.

I'm not so sure your numbers stack up in our case. For a few months of the year we need the heating on for a couple of hours per day - hurrah for insulation and jumpers! When it's cold, we need hot water for showers and the occasional bath. All of our washing appliances are cold-fed.

If I take a look at the last 12 months of energy usage - in a previous flat where we used gas for cooking and had inefficient hot air heating - our usage was:

3294.0kWh of Electricity which cost £357.42
10172.0kWh of Gas which cost £336.73
(Ignoring standing charges and VAT)

If you're right and our roof could generate 3x energy of PV, and even if all of that gas was used for hot water, and all of it was offset by solar thermal, it still wouldn't make financial sense.

And, during the summer, it would be totally useless - except for the occasional need to do some washing up. The radiators wouldn't be on and I like my showers cold. There can be no "feed in" with hot water.

Electricity is just more useful than hot water.

Solar Thermal obviously has its place - and can probably be used in conjunction with PV - but for a modern household it makes more sense to go with PV. Yes, there could be better incentives for hot water - but I think most people would rather have the multi-purpose nature of electricity rather than one-trick pony of hot water.