There are 2 different parts of knife law using 3 inch measurements.
"It’s illegal to:
sell a knife to anyone under 18, unless it has a folding blade 3 inches long (7.62 cm) or less
carry a knife in public without good reason, unless it has a folding blade with a cutting edge 3 inches long or less
carry, buy or sell any type of banned knife
use any knife in a threatening way (even a legal knife)"
Confusing the two could land you in trouble.
https://www.gov.uk/buying-carrying-knives. Worth a read if you're unsure of what is legal.
For those who use knives legally, and either professionally or recreationally, it's important to stay within the law and not be intimidated by the law. Knowledge helps.