You would do this by calling Virgin Media on 150 or 0345 454 1111, taking option 1 initially and then option 4, followed by 5.
This gets you through to the UK cancellations department but .....
BEWARE you would be liable for a large financial penalty for cancelling TV, as you have not met the minimum contract commitment for TV.
You should discuss keeping TV at the minimum level, rather than cancelling.