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 up to service provider to implement such involved checks. Not sure about e.g. Netflix allocating resources to implementing this, clearly resulting in customer loss.
I expect service providers to cut corners to both comply with local laws and not frighten customers away.