The UK has no legal jurisdiction outside its territorial borders -- unless it is a criminal matter and the The International Criminal Court takes jurisdiction -- that is not even a remote possibility. So fines to persons outside of the UK's jurisdiction are not going to happen.
Billers (VISA Net processors) within the UK might be strong-armed to force their customers compliance. With Brexit, the credit card associations could just move to Ireland and out of UK jurisdiction, and into the EU and the Eurozone.
The UK government could strong-arm the UK banks and money transfer agents to cut off payment to individual entities.
There are also new Fintech payment transfer, alternative payment methods as well as surreptitious ways to access and pay for porn that the average 15 year old teenager will learn from his more sophisticated school chums.
That leaves ISP's to be the UK porn police -- blocking sites -- that is not going to be that effective.
In the end, some may lose the majority of their UK customers and their revenues over non-compliance with this UK law -- that's all ...
This may very well make the war on drugs or war on terrorism look like a walk in the park 
Everyone that has tried to go to war with porn has lost -- when there is product demand people will find their way to the product.