There was one advertised in our local paper for £25 secondhand which got me wondering if you could just swap the boxes or whether it was a different subscription.
I'm fairly sure it wouldn't mean a different subscription. Sky Plus doesn't involve extra channels (as far as I know), so I think all you'll need is the box. Just swap it with the one you have.
The old TV still going eh?
Well, when it does go wrong, you might be pleased with the newer ones that have the wide-screen format. This does enhance viewing pleasure in my opinion. The prices of 'HD Ready' TVs are coming down a lot. I think people are beginning to discern between 'Full HD' and 'HD ready' these days. It's full HD that is preferable, hence some good deals on HD Ready TV sets now.
If you are not worried about HD, then an HD Ready TV is perfectly OK. There isn't a choice to have a new TV without some HD capability. HD Ready is all that I have at present. Couple it to an HD capable box via an HDMI cable and it will give a picture that has very good definition on the channels that transmit programmes in HD.
Perhaps the Sky box in question wouldn't be an HD capable one.