Thanks for that! Will see if our local Sainsburys does them. And am so sorry to hear you had to deal with fraudulent card transactions too, it is a total PITA. (In my case it was hundreds of pounds and left me overdrawn, so not going to risk putting myself in that position again!)
I've just had a look about online, Sainsburys don't seem to do them, preferring people to buy a sainsburys gift card. At least I cab find no mention of them doing them anywhere.
Asda and Tescos seem to, or at least a couple of years ago. Martins (newsagents), WHSmiths also do them, or did a couple of years ago (2016). The post office still does some form of 'voucher' which can be used online, anywhere apparently, in place of a credit card but I am uncertain of this as it appears to be a one hit thing from what I've read (i.e. if you had a £100 code and use it the whole £100 gets transfered - damned odd way of doing things so I 'hope' my understanding is flawed)
The system is much more like I remember it than how I've seen it described recently.
Paysafe vouchers is a term that keeps coming up.
https://www.paysafecard.com/en-gb/
(I have not used these, and am not recommending them, it's just what people seem to be using, mostly)
ETA - It appears that paysafe is the company that asda and whsmiths are using for these cards.
https://www.paysafecard.com/en-gb/buy/retailers/
It appears much simpler than other companies, walk up, give them money (multiples of £25 from the looks of it), they give you a card to a preset value. The only downsides seems to be that if you still have credit on the card after 13 months they start charging you £3 a month (so don't lol), a single transaction limit of £750, and they won't let you credit more than £40,000 a year lol. I am currently unsure about transaction fees - it may be £2 a transaction, it may be nothing, it depends on how they view a donation from the Uk to the US.