For all ISPS
Change the default DNS, would recommend OpenDNS (208.67.222.222), Quad9 (9.9.9.9), Cloudflare (1.1.1.1) or others you can check from here :
DNS Speed Test - Find Your Fastest DNS
Would need to check the specific modem/router combination to change the primary and secondary DNS. If it's locked for you then it would have to be per device unfortunately.
For EE :
1) Log in to your EE Account as the primary account holder.
2) Go to your mobile or broadband device settings.
3) Locate Content Lock or Parental Controls.Change the setting to Off and save your changes.
For Virgin :
1) Log in to My Virgin Media - Account Settings -Online Security - Web Safe
2) Toggle both Child Safe and Virus Safe to Off Restart your hub to apply changes.
3) If Web Safe re-enables automatically, contact support or use custom DNS to bypass it.
For Sky
1) Go to the official Sky Help Page for Broadband Shield.Log in using your Sky ID username and password.
2) Navigate to the Broadband Shield settings section.Select Disable Shield or click the Switch Off button to turn off all age ratings and malware/phishing filters
3) Save your settings and reboot router after 10-15 minutes.
Doing this along with a vpn would be best but can these first.