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?
It's conceivable that a VPN provider could change the V6 IP on their server every hour for the rest of time and still get unique addresses.
If the VPN server only has an IPV6 address and no V4 address, can they connect to the target website?
The number of these systems scales in a reasonably tractable way — on the order of the number of ISPs and physical Internet infrastructure around which traffic needs to be routed.
As well as making aggregate routing possible you can use the ISP’s registration details see what location (or legal jurisdiction) a whole chunk of address space has. Hopping around IP addresses will give you unique ones every five minutes but they’ll all still be inside 2001:123::/32 from AS1234 aka Apathetic Onion’s Finest Habidashery and Internet Connections LLC, Delaware, USA.