That £1.7bn profit is because they sell an incredibly high level of units. Not because the markup is high.
If you sell billions of things and only make 1p profit on each on you'll still make a lot of money.
If they decided not to make any profit, you'd barely notice a difference in your energy bills. You might save a fiver over a couple of months.
Let's assume the £1.7bn profit is all from domestic energy supply.
67 million people in the UK. £1.7bn over 67m people = £25 per person, over an entire year. So the grand total of £2 per month per person.
Bottom line it that it's not their profits causing our energy bills to go through the roof.