I assume that you wish to run a VPN client and connect to a public VPN server.
It is not very clear what benefit you expect to get from doing this.
is a discussion about alternative public VPN servers and this is the comparison chart
VPN clients are not very easy to configure. If you can't configure a client on the BT HH6 you might do better replacing it with a different router, or you could have a different router downstream of the HH6 and just ensure that anything that you want to connect to the VPN connects via that router. I have a TP-Link Archer C7 running DD-WRT. It is downstream of my main router and I use it to connect to a private VPN server at another site using Openvpn protocol.