You can via the Priority app https://www.virginmedia.com/virgin-tv-edit/tips-and-tricks/how-to-get-priority-rewards-tickets-offer...
Or call customer services as it's always a high odds lottery as to whether you get anyone good.
You should bookmark the https://www.virginmedia.com/virgin-tv-edit page
It often has competitions for both VM and O2
There's currently a VM Mobile competion linked from that page to win a 65 inch Sony TV with Virgin Mobile