You have avoided tax by buying roof mirrors. Virtually all other goods are subject to VAT at 20%. The govt decided to offer a tax waiver to nudge uptake.
Edit to add.. the FIT for roof mirrors is essentially a tax determined by the govt on all electricity consumers. Costs us all about 0.30p/kWh.
Deffo.. I assume you took advantage of the zero VAT rate on your roof mirror you cad.