its a point of much contest in the world of the installers - many have been so hard hit by the tariff drops that they try and squeeze value out of each sale as much as possible... to hell with common sense. hence these things are being promoted all the time. personally i think self consumption will become an important area in the future, but the tariff system has to catch up with this technology first
solar thermal will generate 3 or 4 times as much energy per sq mtr of roof space, but is under incentivised in the current regime. I would urge you to use your saved £s to go solar thermal!