Hey, I've had similar experiences to you and I'd love to give you an insight into my experience.
I joined virgin 2018 and left 2019 to join Sky who's maximum speed here is 74mbps which is pretty low compared to Virgin's maximum 1gbps speeds available here, joining virgin again through the volt deal this year was a no brainer for me, they even made sure my bill was lower than what I was paying sky. So I'm essentially getting more but paying less, sky could not compete. My speeds are now 220mbps and I pay £17pm in total, £13 for my O2 SIM. I'm giving you this information because personally I believe you're on the best provider if we're comparing sky and virgin. Virgin's routers have double the range, are faster and come with in built automatic ddos and hack prevention.
Sorry if any of that's irrelevant, on your question, from my own experience this can all depend on the advisor you get and how you approach it when your contract ends in 18 months. In my experience it's always good to give their sales team a ring and ask if they'd do you any deals to convince you to stay with virgin, as your contact is about to end, you can also call the cancellation team to do this too. If you've had a better quote from BT, Sky, TalkTalk etc just let the virgin advisor know about it and tell them you're interested in it.
In general, most virgin advisors will check with you to see if there is anything they could do or you'd like to entice you to stick with virgin, as most providers would but I can't guarantee what your experience will be.
I hope this helps In some way.