If it is a brand new SIM card with a brand new number, it should be active and ready to use. We only usually send out SIMs that need activating when it is a replacement SIM, rather than a brand new SIM.
(If it is a brand new SIM and it is not working, it could be worth checking the phone that you are using the SIM in is definitely unlocked, and if the SIM is not "pay as you go", then it might need roaming activated, which can be done either online (see option 1 below) or via customer services (see option 2 below).)
However, if the SIM definitely does need activating, there are 3 ways to activate the new SIM, two of which will definitely work, even if you are abroad.
1) You can register or sign in online at https://mobile.virginmedia.com/ecare/login. (If you are registering for the first time, you will need an activation code that would be sent to the old SIM, so this method would only work if you already have signed up and know your details, or if you have the current active SIM with you.to be able to receive the verification code.) If you can access your online account, go to "Plan and Device" tab, and there should be an "activate now" button on the "summary" tab,
2) You can ring Virgin customer service direct from any country if you can get access to another phone. Just call +44 7953 967 967, and they will be able to put through the activation, as long as you can pass our usual security checks when you call.
The third method I think is designed more for the UK, and involves sending the text "READY" to 789678 . You would send this text from the current active SIM though, so if you don't have that, you won't be able to use this method. I cannot be sure this third method will work when you are abroad, But if you try it, you'll know it has worked as you will get an automated reply confirming that the new SIM will be activated.
I work for Virgin customer services, but I'm on here unofficially - just trying to help :-)