https://www.legislation.gov.uk/ukpga/1988/33/contents
The knife stuff starts here: https://www.legislation.gov.uk/ukpga/1988/33/part/XI/crosshe...
That talks about the blade folding and being under 3 inches (you don't need a good reason to carry it in public), or the blade folding and being over 3 inches or not folding. It doesn't mention the mechanism of locking.
Knives that are illegal would be covered by section 141 - "offensive weapons".
The Knives Act 1997 covers marketing of knives: https://www.legislation.gov.uk/ukpga/1997/21/contents
The schedule to the Criminal Justice Act lists what weapons are currently illegal: https://www.legislation.gov.uk/uksi/1988/2019/schedule/parag...
You'll see item 1 is a knuckleduster. If you're a child and you 3d print a knuckle duster a police officer is likely to tell you to stop fucking about and they'll confiscate and destroy the item. If you're a drug dealer with a history of violent offences they're likely to prosecute. https://www.bbc.co.uk/news/uk-wales-45275791
This is a cultural difference in law enforcement, and it's something that derails conversations all the time on HN. Discretion in charging and prosecuting offences are seen as a good thing in the UK. But it's really important to remember this element of discretion when talking about how the written law is applied.
And indeed, about discretion. So much of how things work is outside the written law, and of varying quality.
> Lock knives are not classed as folding knives and are illegal to carry in public without good reason. Lock knives: * have blades that can be locked and refolded only by pressing a button * can include multi-tool knives - tools that also contain other devices such as a screwdriver or can opener