I agree with Jomifl.
Your TV set is designed primarily to provide a signal via its built-in Freeview tuner. However you want to use it simply as a monitor, to display the signal from an external source, so you need to tell it that.
Press the 'AV Source' button on the remote control. (It's immediately above the '1' button). Use the 'Up' and 'Down' buttons to scroll through the available sources, looking for 'Scart' (or something similar). Then press 'OK'.
However you probably shouldn't be using a Scart cable anyway. Unless it's really ancient, your Sky box should be capable of receiving HD channels. Your TV is capable of displaying them but you'll only see HD programmes in SD (standard definition) if you use a Scart cable. Use an HDMI lead instead!