I'm really not seeing why this would now be a bad thing. There were police right there at the London attack, and if they'd been armed then some lives could have been saved.
If someone drives a truck through a crowd in somewhere random like Sheffield, there will be police there, but currently not armed. How would they stop them? they would have no option but stand and wait until armed response arrived, which in most areas would be a hell of a lot longer than the 5 mins. or so it took in central London.
We're sitting ducks.