It sounds like you're confusing different things.
Fixed vs variable relates to the unit rate and standing charge. Because of the current high price of energy, no-one is offering fixes and you'll sign up to the price cap tariff, which is variable, but won't change until April 2023. It will then go up 20% and again be variable, but be the same for another year or so, depending on whether the government changes its mind on price protections.
How much you pay depends on how much you use (obviously), irrespective what sort of tariff you're on.
There's lots of ways to pay, fixed DD, variable DD, quarterly prepay or pay on receipt of quarterly bill. DD is usually cheaper, but your energy company will have to guess how much you use to set your DD, but if you sign up at this time of year, chances are you'll be in debit until next summer. If you sign up now and choose to pay by quarterly bill, you'll have to pay for all your winter heating in one go next February/March. Can you afford to do this?
There's help with the cost if you're in receipt of certain benefits, but that's based on qualifying at the right time.
Lots of info here:
www.moneysavingexpert.com/energy/
But yes, Octopus are very good.