It does not matter if you have a "HD ready" tv or a "full HD" TV, you wont actually SEE anything in HD unless the data coming IN is in HD.
So if you are watching freeview, or "normal" Sky or Virgin you wont see it in HD.
As Ethel says, to actually see HD you either need:
1) A Blu Ray player with a Blu Ray DVD (dont bother with HD DVD as it is a dead format).
2) A Sky HD box with a Sky HD subscription (which is an EXTRA �10 a month)
3) A Virgin HD box.
4 A "freesat" (BBC/ITV) box capable of showing HD. While you have to pay for the box there is no monthly subscription. You do need a satellite dish though.
More on "freesat" here
http://www.freesat.co.uk/
(Note that rather confusingly Sky also have a service called freesat but that is NOT the same thing)