You have missed the point of the story. GP is not a customer of Ovo energy. They sent him a bill in error.
“Companies cannot just go charge random people”
And yet they do. Anyone can send anyone a bill.
OP said "Ovo Energy made a clerical error and sent me a bill, leading to a dispute." I don't see what a company in that situation hopes they can accomplish without a contract for providing the service. Even if they knew OP's meter number.
Partially "slamming" where agents would try to get folks to switch to a different billing provider, and get paid a commission. So some fraudulently "sign up" random addresses they were supposed to visit.
The other case has been miscommunication over phone or email to someone actually requesting to change billing provider. Or error on the part of the potential customer.
I've had a bill from some random billing provider. In my case it is common for folks newly arriving in the block of flats to get the digits of the address transposed. Due to them using the common English convention, whereas the part of Scotland I'm in uses a different convention.