I expect they will try the rubber hose method first. Not literally of course, they will pick someone who uses a VPN, take their equipment away for forensic investigation and maybe throw in some child porn charges for good measure. Make their lives a misery for a few years, then eventually return their equipment wiped and broken.
It will have to be someone who is innocent, so that people get the message that innocence is no defence if you use a VPN. You will be investigated and your life wrecked, name and face in the newspapers, unemployable and unable to afford legal council.
Unlike many other countries, the UK has no written constitution (despite periodic hand-waving about "Magna Carta"). The UK parliament can basically enact any laws they want. In the past, UK citizens could take a case to the European Court on the basis that a particular law contravened the European Convention on Human Rights. However leading Brexiteers, and even the current Prime Minister Theresa May (a notional Remainer), have made it clear that they want to plug that "loophole".
Makes you proud.