A quick glance down the page in my link shows that Sky Arts is on Virgin channel 165, with Sky Arts HD on channel 140. (Pressing Ctrl and F together, to use your browser's search function, should make it even easier to find).
If you go to the Radio Times TV listings, clicking on 'Freeview' (near to the top of the page) shows all of the Freeview channels numbers beneath the relevant stations (down the left-hand column). Similarly, clicking on 'Virgin' will show the channel numbers for Virgin Media.
https://www.radiotimes.com/tv/tv-listings/