It is apparent some, not all can have issues with VPN use on Virgin Media, where some will say ours works 100% of the time, others say ours doesn't but other VPN's do, theres no exact science in whats going on but VM don't like and don't want you to use VPN's, you must do a few bits of work on your router first...

You must turn off Websafe!

Something to try, we know some of you are affected and others arent...

• 1 - Connect to the router with a LAN cable.
• 2 - Open any browser and, in the address bar, type Virgin Media IP address — 192.168.0.1.
• 3 - Enter your credentials and press OK to log into the router settings menu.
• 4 - If you haven’t changed your password, the default one is placed on the bottom of the router.
• 5 - Open Advanced settings.
• 6 - Disable ‘Block fragmented IP packets‘ (SuperHub 3) or let IPSec, PPTP, and Multicast pass through Firewall by checking the boxes (older Hubs).

• 7 - Confirm changes and restart your router and PC.

This from our research is also a must but a similar description to above

• 1 - Plug in your Virgin Media router
• 2 - Go to 192.168.1.1 or 192.168.0.1 on your browser
• 3 - Add a new username and password.
• 4 - Go to your browser settings and click Advanced settings
• 5 - Select the firewall and tick the following protocols
• IPSec Pass-Through
• PPTP Pass-Through
• Multicast Pass-Through.
• Afterward, click Apply and save your settings