Dammit, killing people isn't a new idea. There's been a lot of thought and work put into it these last few millenia, and there's a good bit of prior art. The experts in large-scale homicide all agree: high explosives are the thing. Messing about with propane and petrol isn't even amateur - it's childish. Grow up and get some bombs.
How many people could they have killed if they'd thrown the silly toys out of the car and just started running people over? Cars kill 3-4 thousand people a year in Britain, and they use two whole cars up without killing anyone. They're embarrassing.
In seriousness, the police do seem to have succeeded in making it very difficult for the bad guys to get their hands on the good stuff, and deserve a lot of credit.