If your TV is fairly recent (and you say it is digital) then you probably do have a HD TV.
A HD TV is defined by the number of horizontal LINES it shows. See if you have the documentation that came with your TV and it should tell you.
A HD TV will show either 720 lines or 1080 lines.
Your TV documentation will show the screen size a 2 numbers with an X in between, such as 1366x768.
It is the SECOND number that defines the lines, so if it is 720 or 768 or 1080 (or similar numbers) then you are OK.
1080 is better than 720 or 768 but I think Sky only broadcast in 720 lines at the moment anyway