An important effect of Christianity's spread throughout Britain was that?

Sounds like a school essay subject...

Firstly the spread of Christianity throughout Britain established a pattern of parishes and diocese with cathedrals and monasteries. It enabled a form of unification across the island which at that time was split into several kingdoms which were subject to invasion by the Danish and Norwegian Vikings.. Ultimately this lead to the ability of the 'English' to beat off the vikings and to eventually become united under one king. The church was responsible for initiating an educational system, it founded schools, colleges and universities, it was the driving force behind improvements in healthcare, christians here opposed oppressive governments and kings and helped establish the balance of democracy which is uniquely British.

In terms of values certain things like fairness, being aware of one's neighbour and their needs, standing up against injustice and so on were inculcated through the church schools, and christian based assemblies held in every school until recently.