Soldato
My 2021 daily average usage was 9kWh electric, 45kWh gas.
0.52 * 9 = £4.68
0.15 * 45 = £6.75
Daily Total = £11.43
Annual Total = £4,171.95
Monthly Average = £347.66
That doesn't include the standing charge which is another £22.50/month, so I can see why they wouldn't be happy with you paying £300, as that will leave you ~£840 in debt after 12 months.
As you've said, you're going to try and cut down, but unfortunately they don't know that so only have your historical usage to go on.