Other option is to ring them up say you want to cancel as your box is old and is struggling and you can’t afford to pay for HD. They may give you a deal or they may just cancel.
I posted on here the other day regarding sky. I bought a new hd tv and our old box in the living room is the basic no sky+, no hd type. There’s an hd box upstairs so that was our ‘free’ one used up.
Called them and the first response was ‘its going to cost £259 and you can’t get another deal’ – thats £199 for the box and £60 installation. Called back 5 minutes later to the ‘how do I cancel my sky’ number and I got the box free. I’m paying £60 installation whihc is fine.