BT broadband has BT Web Address Help enabled by default. This allows then to redirect you to another site if they think you typed the URL incorrectly.
This can cause problems when connecting to a VPN, which is setup to no route normal websites through the VPN. When the URL is rejected your VPN then picks it up as a fall back and tries over its connection. However when these Smart systems are enabled by your isp they will never reject them and always try to offer useful content.