What kind of VPN do you want? Do you want to set up a server so that you can connect to your own home network from another network such as your work so that you can access sites which are blocked by that network? Or do you want to use a VPN provider so you can access sites that BT are not allowing you access to, also are you wanting to do this from a PC or something else.

If you want to get a free VPN on a laptop you can download ProtonVPN which is pretty decent and use the laptop to visit those sites, or if you have a tablet/phone etc you can set your laptop up as an access point and connect the tablet/phone to the laptop which is running ProtonVPN.

Or you can get a router which has VPN software such as an Asus one which has vpn built in:
Code:
https://www.asus.com/uk/support/faq/1008713/
You would set that up and connect your Azus router to your BT router, but your devices would connect then to the Azus router to access the net via the VPN.