[dfawcus]

Given that the English constitution and the Scottish constitution are incompatible (insofar as where sovereignty lies), it is no real surprise that the UK has never got a single codified one.

As it is the Scottish constitution is generally ignored, and folks more or less assume that the English one applies to the whole UK. If there was an attempt to properly codify the UK constitution, that incompatibility would have to be addressed, and that would open a can of worms.