Re: VPN for my plusnet router.
fwiw, Surfshark still supports insecure L2TP protocol, so in theory, any off the shelf cable router (without modem) should work.
Here are L2TP instructions for ExpressVPN and TPlink router for example:
Just plug the vpn router into existing ISP facing wifi router.
If you want to use more secure protocols like OpenVPN, you will need a more expensive/powerful router.
eg. a used Linksys EA6350 v3 with quad core ARM processor can manage 25 mbps openvpn speeds with OpenWRT. Asus RT-AC86u can do 70+ mbps openvpn speeds with stock AsusWRT firmware.