Vodafone had one of these for a short while a year or so back before realising it wasn't a very profitable way to go! Sold with the k3565 modem, £15 topup for each 1GB of data with no credit expiry - however, the SIM will be deactivated if the dongle is unused for several months.
People have caught on to the fact these are rare, so they fetch pretty good prices on eBay. If you managed to find a boxed one, it will have a red circular label '£15 top up only' and 'Top Up and Go Mobile Broadband - 1GB' on the box (red gradient with white stripes).
Note that the K3565 modem was also sold with a 30-day expiring credit, so watch out.