They didn't even have to legally mandate it, "suggestions" were made that certain particularly rabid newspapers would start publishing damning articles about which ISPs weren't filtering out paedophile material. Filters were essentially in place as far back as ~2007.
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In the UK we've had ISPs being forced to block sites for a while.
Fortunately, it provides a handy list of providers to use with your VPN. e.g. https://www.virginmedia.com/help/list-of-court-orders
They didn't even have to legally mandate it, "suggestions" were made that certain particularly rabid newspapers would start publishing damning articles about which ISPs weren't filtering out paedophile material. Filters were essentially in place as far back as ~2007.