It becomes interesting when a company operates in the UK, but does all its software development elsewhere. UK law can't compel foreign software developers not to talk about what they've been asked to do, so I don't really believe the law will be successful in keeping such monitoring secret.
Even more interesting is how this would work regarding open-source software. You can't keep such a backdoor secret in GPL'ed software and comply with the license, but a UK company couldn't release changes they'd made to comply with the law either. So if you suddenly see UK companies move away from GPL'ed software for no obvious reason, that may be a clue.
Whilst you can't compel foreign software developers not to talk about what they've been asked to do, there are probably a couple of ways this remains possible:
(a) Disclosure would likely result in liability for the executive officers and/or Company Directors, which might be contempt of court, or something more serious;
(b) Software developers working for Apple (an example) are likely under strict Non-Disclosure Agreements as part of their employment contracts, so the company served with such a request will likely make it clear to those responsible for technical implementation that it's covered by NDA.
Even in the case of (b) if a developer quits rather than implement the functionality, in many jurisdictions, the employer would have grounds to pursue a gag order.
If I quit a job at Apple over this, Apple might have to sue me for appearances sake, but my GoFundMe account would be quite fattened by the experience.
Even more interesting is how this would work regarding open-source software. You can't keep such a backdoor secret in GPL'ed software and comply with the license, but a UK company couldn't release changes they'd made to comply with the law either. So if you suddenly see UK companies move away from GPL'ed software for no obvious reason, that may be a clue.