I know the feeling. If you're still in contract it might be tricky and there could be a price to pay, but if you've been with them for a while you might be out of contract in which case it's just a matter of giving them notice.
If you telephone them, record the call if in any way possible, and ask them to confirm in writing. If you can do it via webchat, capture the screen.
If your experience is like mine, they will make it as awkward as possible, and I'm afraid they can't be trusted not to lose track of it.
Don't throw any of your kit away in the meantime, and when the time comes to send that back, make sure you take a picture or video of what you're returning as evidence.
Last but not least, as soon as you've asked to cancel your contract, make a Subject Access Request. This, by law, obliges Virgin to send you a copy of all of the data that they hold on you. They must, by law, fulfil it within 30 days or justify why they can't. The point of this isn't to be vexatious, it's to get written evidence of everything you've gone through with them. I can't recommend doing that strongly enough.