I would like to see one shred of evidence that having so-called backdoors would have prevented the London tragedy.
Maybe if some wacko Eagle Eye supercomputer were monitoring and evaluating all communications at all times in real-time, maybe something could be sniffed out, and on that day we shall all bow down to our robot overlords. Until then, you're talking about creating a mile-high haystack of data, and hiring humans to eaves-drop and search through it day and night for a needle. I mean, politicians really need to think before they speak. Already, London has more cameras than nearly anywhere else on Earth. The problem? If you're intent is to prevent crime, you need eyes watching all those cameras all the time. And there's nothing more boring than looking at security cameras all day.
For catching and prosecuting a guy who's committed a crime, maybe. Go back through the tape, catch him in the act, conviction, sentencing. Done. But for preventing the crime in the first place, particularly where the perp is not planning on walking out alive, backdoors and security cameras don't justify the legions of personnel it takes to watch and listen to everything just in case someone might pick up the perp maybe saying they're gonna do something. And who's to say it's real, or just some dumbass talking shit after drinking too much? An overworked, underpaid government-contracted data grunt, that's who... he's the reason the storm-troopers in riot gear tore your house down, zip-tied your family and shot your goldfish, only to find your 10-year-old pulling pranks on his iPad.