The guy on the end of the phone might well be working on a 'commission only' basis. He gets a fiver if you'll accept an appointment for a rep to call at your house, which is doubled if the rep gets you to accept solar panels. The guy on the phone couldn't care less whether you'll actually benefit from solar panels; he just wants his five or ten pounds, so he'll tell you anything to get it.
Similarly, the rep who calls might be on commission only, perhaps getting £40 if you agree to take the solar panels. Again, he doesn't care whether they'll actually work or not, he just wants his money.
Once you've agreed to take them, the solar panel company will happily take your money (or the money forthcoming from a grant, as appropriate), again with no concerns about whether they're actually providing you with a useful product or not.
That's the way that it works. (Please believe me: I've worked as a rep, on 'commission only' basis, and hardly covered my expenses because I was honest and simply couldn't sell people a service they didn't need and couldn't possibly benefit from. Other reps for the same firm made good money, with the majority of it earned by selling to people who couldn't possible benefit from the service that we were offering).
As VHG suggests, any 'wonderful' product or service offered by a cold caller should almost certainly be avoided.