I work for one of them, and it's not that simple. All buy their fuel from the same traded market, so the price of gas does not reflect the price of getting it out of the ground, but how much the selling traders can get for it. So this, approximately 50% of the price you pay for the gas you use, is dictated by the capitalist market and as gas is making up an increasing share of the fuel used to generate electricity it directly affects that too. To manage the price to some extent it is bought up to 2 years in advance of use and typically 6 months to 3 months ahead. Buying on the 'spot' market and charging customer on that basis would be practically impossible, (and massively expensive).
The other 50% of the bill is made up from costs largely levied or fixed by the government/regulator for delivery through pipes or wires, wind farm subsidies, VAT and lots of other charges which your energy company has very little control over.
So, despite what the papers would have you believe, and the government appear satisfied to not correct the view, I don't believe that price fixing and collusion is happening. It is I'm afraid a consequence of how the deregulated, capitalist market works, (now there's a thread that would run and run). We all reaped the benefit of this for many years, (yes, including the energy companies) but now deregulation, exposure to commodity markets and the costs generated by environmental concerns, (etc.) is not working in our favour.
Having said all that, I can't explain why a few of the 'big six' have managed to lower the electric price, and not gas.
edit - couple of other things - yes, most extract the gas, trade it and buy for onward delivery all in the same mega-corporation, but the rules of the market mean that these parts of the business are entirely separate legal entities, so selling themselves the gas based on the cost of extraction would get them busted. They have to trade it through the same big market to avoid the collusion they are being accused of.
Oh, and I do buy shares though our share save scheme. I'd not recommend it!!