I'm the wrong person to try to steelman this, but lets have a go anyway with a couple I know well:
1. Internet Service Provision
Britain chose to separate the intrinsically geographic monopoly of Last Mile Copper Loops from its existing non-intrinsic monopoly telephony provision. British Telecom, which had once been publicly owned, was obliged to distinguish "BT Openreach", the part of their company which inherits the local loop monopoly and which also offers (non-monopoly) long distance network transit and other useful stuff - from the consumer facing BT, which is a fairly ordinary PLC. I know a BT employee, the regulators really care that they can't collude with Openreach.
This means that the tiny company providing my Internet access (Andrews & Arnold) doesn't need to own a large brick building down the street, or even a cabinet in that building, or negotiate a deal with a monopoly behemoth who can set their own terms unfairly. Instead, they pay Openreach to move packets from my home to a nearby city, and then they can choose to pay Openreach or its competitors to move those packets from the city to their routers. BT Openeach is a monopoly, but it's an ordinary public company just with a lot of regulations to ensure it behaves equally for A&A as it does for its owners BT. BT isn't a monopoly but it does have lots of customers, I think its services are crap or alternatively that they're too expensive (A&A is more expensive but much, much better, if BT were much cheaper than they are maybe that's a good deal for somebody)
This arrangement means most UK residents have dozens of potential reasonable ISP options, with a range of pricing and terms, including lots of "All you can eat" type packages, even if they don't live in a big city. People like me who do live in a big city get slightly more options (a Cable TV company, a local fibre startup), which can go cheaper and higher bandwidth, but only a few people are stuck with a single practical option as is common in the US - basically only people in very rural areas, and usually their option is community owned, so it may not be cheap but it's at least owned by actual people who might care.
2. Energy
Britain separately privatised the gas and electricity supply. Now on the surface this is lunacy because of course that's a geographic monopoly. But they're not complete idiots, so what was actually privatised was mostly the customer service/ end user billing part of the problem. My gas and electricity come from the same place as my neighbour, inevitably because they're the same pipes and wires, but my bills and my customer service are from my choice which happens to be Octopus.
Does this achieve anything useful? Eh, maybe. I don't think a local energy monopoly would be anywhere close to as good at either customer service or billing. Octopus seem to have some idea what they're actually doing. On the downside these firms have ended up costing tax payers a bunch of money because of course when one goes bust the gas and electricity are still working but the government is on the hook to ensure somebody else handles billing them for it and that's complicated. This happens far too often when there is stress on the financials of these firms e.g. the Ukraine situation fluctuated energy bills with little notice.
Edited: fix s/citizen/resident/ what matters isn't your passport but where you live