One tweak to the rules and all of a sudden not only do porn sites have to verify the age of their UK visitors but also anyone connecting from something other than a residential ISP.
The more troubling thing about these laws is enforcement. The threat of fines only works against websites that map to a business entity. For anything else there will surely see a ramp up in the size of The Great British Firewall Ruleset, edited by the courts, and distributed to the Big N (5?) ISPs.
What will become of the smaller ISPs that refuse to block illegal sites?
The UK instructs ISPs to block access to deliberately non-compliant sites, however users want to make connections to the sites and those sites want to receive connections to those users. VPNs will be effective in allowing access to non-compliant sites as long as ISPs can't identify the VPN traffic.
Of course, the British ISPs can initiate the tactics used by China to identify and block illegal traffic. However there are limits to this. Unlike Chinese users, British internet users regularly make connections to international servers so various bridging techniques are possible. Like VPNs, proxies or even Remote Desktop.