Given recent polling, we have to assume that what is MI5's today will be Reform's tomorrow. We have to ask what our government and judiciary are doing putting our privacy in the hands of the far-right.
Does it matter who has access to the data? It’s the principal not the actor
Every encryption backdoor is a huge vulnerability. Even if we somehow ensure that the powers-that-be remain entirely trustworthy (something that, historically, we can't even manage for a century), they're not the only people who'll have access to the backdoor. It's not possible to make an encryption backdoor that only authorised parties can use: as they say, the laws of mathematics do not respect the laws of Australia.