If you are connected to the internet network provided by Vodafone or BT (Wi-Fi, Ethernet or Mobile Data), and can not get connected to Surfshark servers, that could be caused by content filters. That especially applies if you are using a router provided by the same company.
Since there are many different routers available, settings might be different. But in general, you may want to log in to your router, set manual DNS servers (for example 220.127.116.11 and 18.104.22.168) and also look for some security controls or parental filters. If you find any "Safe Content", "Parental Controls", "Secure DNS" or similar features enabled, turn them off, reboot your device and try to connect to the Surfshark server again.
If you are unable to find any restrictions on your network but there is still a problem with the Surfshark VPN, feel free to contact our Customer Success Sharks and they will help you out!