Hi missprim
The easiest and cheapest way of watching HD is to purchase Freesat HD box �149.99 at Argos, However there are not many programmes yet broadcast in HD mainly BBC1 & 2. but it is great to watch. There are no further costs such as monthly subscriptions.
The only other way to watch is very expensive, you have to get Sky to install a complete system with dish & HD Box then pay �10 per month for the priveledge, plus if you want the full range of HD programmes such as sport and/or films your subscriptions will rocket.
Hope this helps. johntaylor99