Not sure if this will help you, a friend of mine rents out a villa and has a UK bought card meter, I have seen it fitted in the house and it sure does look as if he fitted it in, I did remove the main fuse box cover once and you can see where the card reader wire goes into the main fuse box and the chasing out of the wall to cover the cable.
He has a chap care for the house and should any visitors need extra prepaid cards he hands them over for 10.00€ a card, UK meter...Have to presume the cards are £££'s as never seen one, the display reads the amount left in Sterling, must sound a bit weird I know but it all works, where the cards are supplied from I have no idea, the meter probably E-bay.
Hope this helps in some way.