See
https://www.virginmedia.com/help/leaving
The address is on there so you can just pop a recorded delivery letter into you local Post Office if you don't want to phone.
Write to Virgin Media, Sunderland, SR43 4AA, including your name, address and account number.
If it's because of the 2023 price increase AND you are in contract, then mention you are using the Ofcom rule on contract exit without penalty because of price rise and ensure its received by them before 30 days of the dated letter or email sent by VM.
The forum staff can't process cancellations on here.