Years ago residential customers in the UK were obligatory customers of monopoly government owned suppliers in each region. You could only buy (residential or small business) electricity from the same people who ran all the cables where you lived, thus a natural monopoly, which was government owned.

Then there was an era of government very enthusiastic about the Free Market even where it has no obvious place (again: natural monopoly) and even more enthusiastic about moving debts and long term costs off government books created a new system. The actual power grid - the natural monopoly - is now run by a single for-profit company but the consumer buys electricity from any of dozens of pointless entities that exist only to buy and sell notional electricity and bill customers on top of that supply network, many created from the carcasses of the old monopoly regional suppliers.

To bootstrap this situation consumers needed to start out with a supplier, and so they were all assigned the one that had previously been their monopoly supplier anyway, the "incumbent".

What was expected to happen was they'd all offer lower prices to new customers (this happened) and everybody would switch and create a new equilibrium in which almost everybody got lower prices and was more willing to change supplier. Your actual electricity supply would always come from the boring reliable (but now privately owned) grid, but you'd pay whichever imaginary "supplier" charged a few quid less, or had a cool mascot in their TV ads or whatever.

But almost everyone ignored it. Given the choice between paying an incumbent supplier £100 or finding out how to change supplier to get a cheaper £80 deal, almost all consumers just kept paying £100. If you can't leave (e.g. often short lease tenants even if the law says they can change supplier find that in practice they really can't) you're stuck paying £100 while a pro-Free Market politician struts about on TV saying how they've saved you £20.

But the biggest impact is that because the incumbents control so much of the market and consumers don't care, you can offer almost anything, even seemingly unrealistic things like "100% Renewable electricity for all customers" and know you'll only get a handful of takers anyway.

My mother switched to a tiny local "supplier", which promptly went bankrupt. Despite the fact that her power worked throughout (like I said these suppliers don't actually supply anyway) and she wasn't billed for that power (the bankruptcy appears to have made it difficult to even figure out what customers owe them) she found this traumatic enough that she now refuses to ever switch away from her incumbent again.