When I was 17 I took AS level politics, it was possibly the most unpopular subject in school, my most abiding memory of this class was talking about constitutions. We went into great theoretical depth about the type and purpose of constitutions before discussing whether or not the UK should have one. Everyone just assumed that having a codified and entrenched constitution (aka written constitution) was a far off fantasy that served no real purpose; we hardly even bothered mooting the positives of such a document.
The Scottish independence referendum has made it abundantly clear that the UK will be changed forever. A central plank of the Yes campaign was the writing of a new constitution. The very fact that no one, on either side, questioned the need for an Independent Scotland to have a constitution shows that people in the UK aren’t uniquely incapable of grasping the case for a constitution but rather the fact that things seem to be OK right now so why bother? There is, simply put, no impetus for a UK constitution.