You will be at major risk in the UK carrying any bladed instrument up to and including scissors and multitools, foreigner or not, if you are searched by police for the right/wrong reasons. They have gotten fairly fanatical and aggressive towards knife crime considering the coverage and stigmas. There have been a number of laws passed in the last 20 years that are pretty damning.
Also - legislation changes based on location rather drastically. You can be guilty in one municipality and not another. This completely invalidates everything else I'm about to tell you, because local laws trump it.
1- No blade over 3'.
2- "Good reason" to possess. Subject to interpretation by police.
3- Carried only to and from places where you have a good reason.
4- Don't open carry.
5- The locking mechanism thing is unclear. Many police believe your sub 3' pocket knife needs to be non locking in order to comply. Whether this is actually true or not is up to debate, in the courts. Stick with no lock.
(A Crown Court case (Harris v DPP), ruled (case law). A lock knife for all legal purposes, is the same as a fixed blade knife. A folding pocket knife must be readily foldable at all times. If it has a mechanism that prevents folding, it's a lock knife (or for legal purposes, a fixed blade) The Court of Appeal (REGINA - v - DESMOND GARCIA DEEGAN 1998) upheld the Harris ruling stating that "folding was held to mean non-locking". No leave to appeal was granted.)
You will get arrested for box cutters, screwdrivers, scisscors, kiridashi, sharpened toothbrushes, and any other pointed or bladed instrument that is greater than 3 inches, a fixed blade, or otherwise seems like your carrying it around to potentially stab someone with. Self defense is not considered a good reason.