to CF. I'm pretty certain all PACE STBs in the UK belong to the cable companies, in which case the box you have been given is technically 'receiving stolen property'.
Also, AFAIK, your subscription is tied to the box, not the card, so moving the card between the two won't help. Your best bet, if you want to be able to watch the TV from different rooms, is a Digisender.
[Edit] Beaten to it again.