Sounds like POP3. By default though, the iPhone does not delete messages on the server though. Which mail client are you using on the iPhone?
Looks like Virgin Media needs explicit settings to download POP mail on more than one device.
My suggestion is that you switch to IMAP mode for your email. Instructions here.
It's quite a read. I'll quote the short version, but you should read the link for all the details like, "What is IMAP" and the new IMAP settings for your specific account.
You can retrieve your Virgin Media Mail messages with a client or device that supports >IMAP, like Microsoft Outlook or Windows Mail.
To enable IMAP in your Virgin Media Mail account: Sign in to your
Virgin Media Mail account. Click Settings at the top of any Virgin
Media Mail page. Click Forwarding and POP/IMAP. Select Enable IMAP.
Configure your IMAP client and click Save Changes.