The cable box is part of the service, if you aren't paying for cable service, you won't have any use for the box, except to steal service. That is why you can't own the box, it's programmed for the channels you pay for. I don't watch enough TV, nor am I thrilled with the billing policy, so never had cable. If you search the internet, there are probably places that still sell cable boxes, and SIM cards, maybe you can find something that will work. Since a lot of folks want what other folks have, without paying for it, most likely a cable box forum or two out there. Good start would be to enter the model number of your cable box, or one of your neighbors, that fits your needs into Google, which should give a list of links.
Nigel, why is a VGA monitor unsuitable? Been using a 19" LCD computer monitor as a TV since last spring, when the old 55" arced, sparked, and died. Only problems are that it's tiny, compared to the TV is sits on, need separate speakers/amp, and noticed one night, that back light wasn't turning off with pushing the power button, but it's an old monitor. Using it, because it happened to have a composite input.