the reason the uk wants an encryption backdoor is because it's expensive to do statistical analysis of encrypted traffic. there's ways to make it more difficult, but if you own the certificate that a tls endpoint uses you can just open it and reencrypt it for the destination. this is called break and inspect. if a vpn uses different certificates and is built well, there would have to be a flaw (spyware, vulnerability, etc) on one of the endpoints for anyone other than you and the vpn to read the encrypted data.