Which mortgage companies, where's that written down?
The tarrif route is at the mercy of the government, not if they reduce it but when and not a route affordable to many people. Frankly I've got better things to do with £12k than prop someone up and be left with ongoing maintenance laibility. As an installer you're bound to have a different perspective.
Reduced electricity costs for no outlay is the no-brainer afaik (speaking as an informed customer ).