Every major British ISP has been required by law to insert devices into their core network to enable certain agencies to monitor Internet traffic. The reason most people haven't heard is that they weren't allowed to tell you, and there are reports that people at ISPs were asked to sign the Official Secrets Act concerning their installation.
This is why "canaries" are used on certain websites. If they are ever forced, under threat of imprisonment, to comply with certain agencies, they stop posting certain signed updates that say things like "We have received no such requests from law enforcement today". No updates - they've been compromised and are unable to talk about such things. (Though I'm sure, even there, there would be some way to ensure such canaries continued to be posted even without the original person's co-operation, it would be a lot more difficult than just holding a gun to their head).
The UK is no different in this regard to any other state entity. Nor the EU. The US is actually a lot worse than those, but probably not as bad as China.
Anyone who thinks that ANY government couldn't simply demand that a major corporation on their territory give them total access to their entire system is an idiot. And revealing it, whether on the nine o'clock news, or in an a disguised tweet, would probably be seen as contempt of court or worse. We are the country that had the scandal over super-injunctions, you know. And it wasn't that the country protested and they were abandoned and never happened again - it was years of quiet court action, eventual use of parliamentary privilege and no clear end to the possibility that it could be done again which are the only reasons that we know about them.
The biggest case about such things is the US vs Microsoft case where the US wanted to raid EU datacentres but the EU (and Microsoft EU) said no. If the EU said yes, do you think they wouldn't be demanding access to those datacentres? It was only the "then handing that data off to the US" part that troubled them.
If a government want to put in a backdoor into a product made, you can be sure either a) it will happen, b) that product will never be made (because the creators object to doing so). It's highlyu unlikely c) that the creators will blab about that to the world's press and give themselves some decades in prison by doing so.
P.S. It's only because of government backdown and that you were able to legally discuss it that things like Clipper chips never met mass adoption. You have no idea if, somewhere in that process, something did get mass adoption but was never public knowledge.
I'm the absolutely antithesis of a conspiracy theorist. But if a government want to do this, it'll happen. Strangely, your biggest champion in this area is Apple who are quite vocal about not co-operating with the FBI etc. to decrypt their own devices. It's seen them in a lot of hot water and legal costs, but they happen to basically have the wealth of a country all their own just sitting in the bank to fight such things. You can be sure that places with less funds, more government ties, or less morality have already co-operated and you'll probably never know who, when or what until it does become public knowledge (and therefore too late as it'll be useless to use so they'll move onto something else).
The NSA were attempting backdoor-inclusion into open-source encryption standards for years, and they still have questionable intentions (they have "chosen" the curves that Elliptic Curve cryptography standards use, in many cases - are they doing that because they know that those curves are strong, or that they know that those curves are weak? History isn't on the "protecting the public" side).
To think that any government is above such actions is to severely misunderstand any modern government. These people choose whether or not to murder tens of thousands of civilians to "stabilise" countries, enforce government on them, while selling them arms, buying up oil rights, etc. There are vanishingly few "moral" countries around.
In that respect, sure, you don't want Chinese kit spying on it. But neither do you want Russian, Indian, British, American, Australian, French, German or any other kit spying on you. And those are just as, if not more, likely.