VAT on power bill

[Mercdriver]

Checked latest bill from EDF to see that they calculated the bill then deducted the government discount.

I would think they should deduct the discount then calculate the bill to avoid paying VAT on the discount.

Wrote to MP and surprise surprise got a typical reply from a politician which does everything except answer the question.

Why should the deduct the discount after calculating the bill, anyone on here experienced on VAT who can give me an explanation