You can get HD via the "freesat" service from the BBC & ITV.
You need to buy a freesat HD box, but after that there is no extra monthly payment.
You also need a dish on your external wall. More here
http://www.freesat.co.uk/
You can also get HD from Sky (or Virgin if you live in a cable area) but you have to buy a "HD box" and pay extra each month.
You can also watch HD movies if you buy a Blu Ray player (like a DVD player) and buy or rent Blu Ray movies.
Remember, what you bought was a "HD ready" TV, so it is a TV that is ready for HD. It did not say you were automatically going to get HD.