Because the UK is only borderline a free society.
It's like asking why does the king of Saudi Arabia need to have the power to jail his subjects for no reason? Why does Putin need to have the right to jail or kill political opponents?
The UK doesn't give you right to remain silent (your silence will be used against you), you are compelled to bear witness against yourself (you must surrender passwords), and there are super-injunctions where you are gagged and not even allowed to discuss the legal issue with your lawyer.
This is the country that sent government thugs to force journalists to physically destroy their own laptops and hard drives.
With religious clergy overtly and explicitly being part of government power (in house of lords).
Hell, it's super recent that government power in house of lords stopped being inherited.
Imagine if the US senate was staffed solely by inherited power. Not just in the style of Bust Sr/GWB, but actually inherited. It's not the same, since the US senate is more powerful than the house of lords, but "it's complicated".
So your confusion here may come from the fact that you look at the UK as "like the US, but they talk weird", where it's a couple of step closer to "Like Saudi Arabia, but part of political power is elected".
Obviously Saudi Arabia is much further away along this spectrum, but I hope you see my point anyway. You seem to be saying "how can a free society do this?", where the answer is "because it's not that free, your assumption is flawed".