I'm not sure what law could be used to criminally prosecute Reform Party leadership. I guess without a Constitution, Parliament could pass a law making the Reform Party ineligible to be on the ballot and making it a crime to lead it. But that seems like political suicide.Without a written Constitution, Parliament can do pretty much whatever it wants.
Sovereignty in the British system lies with the "king-in-Paliament." The King has not vetoed a bill since the early 1700s, so the sovereign is pretty much whatever Parliament says it is.
There are a number of ways to go about this:
Legally prosecute the leader of the party (sending a not so subtle message to the second in command, "Okay, Mr. 2IC, if you do not tow the party line, you're next." (e.g. France)
Ban the party. (Germany, maybe?)
If your party is polling poorly before an election, just announce you will not hold elections (Romania, UK)
I saw British polling that indicated Reform would win 500 seats if elections were held today. Now, elections will not be held today, but I think Labour, Tories, and LibDems are worried.
Of course, the conventional parties could resolve this easily: tell the European Court of Human Rights to stick it in their ears. The ECHR is the main culprit. "Human rights" sounds innocuous enough. Who could be opposed to human rights? Until you find out that "human rights" means uncontrolled immigration and housing at the expense of the taxpayers in the countries to which they immigrate. In country after country, folks from Africa and the Middle East flock in droves, burn their passports at the border, and claim "asylum." Asylum in reality means, "my home country sucks and I could never get a job there that will pay one-tenth what a job here does, plus, the electricity works here, the roads are paved, the cops are mostly honest, etc. so I'll stay here."
Any party that gets a handle on the uncontrolled immigration problem will be popular, but the conventional parties refuse to tackle the problem. Reform, AfD, Rassemblement national, have all announced they will try and that is the source of their popularity. The conventional parties have a choice: get immigration under control or just ban the immigration-skeptic party. They have chosen the latter.
If the Reform Party would win 500 seats if elections were held today, that's huge popular sentiment. You might have the legal authority to "cancel" the party. But you can't cancel what the voters believe they need from government.
BTW -- I checked, and there are 650 seats in the House of Commons. If Reform currently leads in 500 of the districts (or whatever the UK calls the areas that MPs represent), that's over three-quarters -- a massive total.
And while it probably wouldn't hold up through a real election, they could lose down to 326 seats (a decline of over a third from their current polling) and still hold an outright majority in the HoC.
The British people are speaking loudly and clearly on this point, and I'm thinking the concern from the Tories, Liberal Democrats, Greens, and Labour is mis-placed. They shouldn't be concerned so much about the Reform Party itself as they should be about the popular sentiment that's driving Reform's recent successes.
If Parliament removes the Reform Party from the ballot and makes it a crime to be in Reform Party leadership, but the population still holds the ideas that drove it, another group will spring up in pretty short order. Would they dissolve and criminalize that one too? And the next and the next and the next?
Whether the politicians want to or not, the people of the UK are demanding that they fix the immigration issue.
I didn't even know the European Court of Human Rights existed. But I agree 1000% with your assessment. Who could be against human rights? Until you find out that that means accepting all comers and providing them living quarters and financial support for an indefinite period of time.
Yeah, especially since the UK is already out of the EU, I'd tell the ECHR to pound sand.