The wholesale rates are not in fact fixed, there's a wholesale electricity market. It runs in half hour blocks, there are estimates of what's available to be dispatched, and what use is expected, and the market seeks a price which will be acceptable to enough of the cheapest dispatchable producers to get approximately the desired energy, then that's what everybody is paid for the energy - it's a bit like a Dutch Auction.
As well as the short term energy market, "suppliers" like Octopus also hedge by purchasing contracts to supply electricity far in advance. In fact the bulk of electricity is purchased this way, which makes that half-hour short term market look more volatile than the reality of energy trading. It also means that to be a serious player you need a lot of capital to make your purchases earlier and with better data than your competitors.
The existence of numerous consumer suppliers is, indeed, pointless, it's just Tory orthodoxy which causes these to exist, you obviously don't need the suppliers since they just basically shuffle paperwork. But there really are wholesale prices, and so there really is a wholesale energy market, even if there was a single government consumer supply company, it would be buying energy from the market, at the market price. Unless you want the government to also own all the power generators, and all the other countries we are buying energy from, which I think those countries might not agree to.