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 does not have jurisdictional power over anything outside their country - they can not a foreign site to do age verification of foreign residents.
Now, the UK can say that they need to check for all UK residents, regardless of them using VPNs. But if there are no practical way to do this, I think the UK will have diplomatic issues enforcing anything to non UK companies breaking that laws - as they would need, eg. Germany, to help them enforcing the law on certain providers.