Knife possession is legislated differently in public places and in private. In private, you can own any type of knife except automatic knives (flick knives, butterfly knives, and switchblades). In general, knives carried in public places are legally considered to be offensive weapons and the carrier can be charged with "possession of an offensive weapon". It is however legal to carry a knife if there is a bona fide reason to do so for example, if it is a tool required for ones trade (e.g. chefs) or if it is part of a national costume (e.g. sgian dubh), or if it is carried for religious reasons (e.g. Sikh Kirpan). A special exception exists for penknives (pocket knives) which are legal without reason for possession, but they must be non-locking and sub 3". Any other article with a blade or point is illegal to possess in a public place, except with a reasonable excuse. [2]Even a folding pocket knife of less than 3" may still be considered an offensive weapon if carried or used for that purpose.
Possession of a 5" bladed knife in public would be an offence.Your reason for taking out may not impress the police that it was for a legitimate reason.
The fact that you cover it as described would make no difference