The way I read it, the subsidy for poorer households is currently baked into the electricity bill. So the value of the subsidy is slightly less than the headline figure, since the cost of the subsidy is shared out equally amongst all customers (including those receiving it).
By moving it into general taxation, then due to progressive taxation none of the cost of the subsidy falls on the poorer households... or indeed most UK households.
Instead almost the entire cost of the subsidy will fall on - you guessed it - the few % of the population who the government hate aka the broad shouldered.
God I've come to hate that phrase.