From a legal perspective, my lay-man's reading (I am not a lawyer) of the T&Cs mean you can do what you like as long as it is legal, doesn't adversely impact other internet users, and doesn't interfere with Plusnet's operations. So using a VPN within those limits should be fine (I use several different VPNs for several different purposes).
From a practical perspective, Plusnet are unlikely to be able police any use of a VPN anyway. Many people use VPNs to connect to employer's networks (I do) which is entirely valid. Plusnet couldn't look inside a VPN tunnel without a court order (even if it wasn't encrypted), so there's little they could object to unless you are connecting to a server which is only used for illegal purposes - in which case it would be likely to be blocked by a court order anyway.
You just have to judge whether your intended usage would be considered illegal (in the UK).
Thought for the day : Laws only impact law-abiding people.