I assume that you're referring to roaming for calls/texts?(Not just data roaming).
If you already checked that roaming is set to on in your account (and International call baring to off).
I'm afraid you'll need to contact Virgin customer services to ensure that it's enabled their end.
You may be able to use the web chat option to speak to an advisor starting from here: Help and Support - Contact Us > Mobile > Mobile not working > Chat online.
The advisors are often busy though so you may not get connected straight away.
If you can borrow a phone to call Virgin Mobile their UK support line number is: +447953 967 967
If you want to say thanks > click 'Kudos'.Answered your question? > click 'Mark as Helpful Answer' also.