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.

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.

You can still be on the camera of the shop which sold you the sim card if they recorded the serial number.