Some are not really pre-paid, you have to go online to activate them, or have to order them online, or have few options but to top up by card.
However, I just looked into it some more and it appears you can still buy prepaid cards by cash and buy top-up vouchers with cash.
You can still be on the camera of the shop which sold you the sim card if they recorded the serial number.
Yes, you can get and top them up with cash, at some dinky corner shop with non-functional cameras. Or just buy a bunch of them on Gumtree, also in cash.