It's always problematic to comment on something that you're not directly affected or involved in. As far as I can tell it is largely a London problem, and at that it's a problem in areas that already have social issues. The knives used have been largely cheap kitchen knives anyway, and when it became more difficult to use those Gangmembers moved to using acid as a weapon. The Mayor is simply doing what a lot of politicians seem to be doing: Put bandaids on something that would require more work something doesn't immediate come with good PR from the majority of the people who don't look further into an issue and just go by what the Daily Mail/Sun tells them.
Knife crime as a whole is a problem in Europe here, in part because a knife can be used as a weapon and is an easy go to choice for criminals while at the same time it's simple something the news can easily latch on to simply because 99.99999% of the time a knife is used as a tool but when it is used as a weapon in a crime it is so unusual that it is news worthy. Since nobody would read a million articles on a million websites reading "Men cut carpet to size using boxcutter" or "Neighbor opens package with swiss army knife". But that once a month incident about a stabbing that gets clicks.
As people said it before, knives aren't the problem, the people using them wrong are the problem, and solving their problems would be a lot more difficult than just banning the tool of choice.