After not seeing 3V cards for months in my local tesco, this evening I saw the pay.com versions. Basically they look exactly the same in design to the 3V ones, but say pay.com where the 3V logo used to be. There is a link on the packaging to pay.com/gift which relsolves to this site where you can see what they look like.

https://www.3vprepaid.com

Didn't buy any before anyone asks.