Firstly Stewie, you're not allowed to carry a steak knife on the streets or into school. Secondly, if we are going to make laws based on who is or is not more likely to carry out crime we will get into a filthy mess. Imagine if there were special restrictive laws for young black men for particular types of crime. Look at the controversy over ethnic profiling at airports, where the 90-year-old Caucasian New Yorker is considered as likely a suspect as a 25 year old Yemeni man. We don't do that Stewie -- it's considered racist.
"Well it allows a "blanket exemption" for lots of reasons, basically any "good reason" you can think of."
No it doesn't. Self defence is a good reason, for example. It does allow a blanket exemption for religion. Why do you think this is good?
"It doesn't say "Sikhs can carry a knife but no one else can."
It does mean Sikhs are the only people who can carry a knife not being used for any function, day or night.
"which seems sensible to me"
Why do you think it is sensible for a knife to be allowed for religious reasons?
Anyone with a "good reason" has a "special dispensation."
Sikhism is the only religion which requires carrying a murderous weapon in everyday life for no practical function. They have special dispensation.
"It also means that anyone dressed like a carpenter can carry a number of truly lethal implements."
Carpenters' tools have a practical function.
"Or dress as a cub master and wear a camping knife."
Practical function.
"Or dress as a camper and walk around the woods carrying an axe."
Practical function.
"All of which is a lot easier than dressing up as a Sikh."
Actually, not at all, particularly if you are a Sikh.
"Religious reasons are simply one of unnumbered reasons as to why you can "carry a murderous weapon with impunity."
But it is the only one which serves no practical purpose.
"If someone is determined to try and "trick" the Police there's so many other ways they will do it, doors that would have to be closed before you began to worry about being in disguise as a Sikh."
If you are a Sikh will malign intent, it will probably the first way you will do it.
And you would then have to work out how to specifically word the law so that religion is excluded as a "good reason".
How's this: "Religion does not constitute a good reason under the law for carrying a knife".