The difference to me is the proximity (time-wise) of "living in london" and "getting married"
a) seems to suggest that the person moved after getting married, the following statement would probably be about where they relocated... 結婚する前に(or even まで)ロンドンに住んだ
b) seems like they lived in london once in the past, perhaps long ago. More like ロンドンに住んだことがあった. This feels a little strange because "having lived in london" feels disconnected from "got married," unless the context is talking about places to move, and someone is saying "yeah, I lived in London once, before I got married (insinuates a long time ago?)... I wouldn't want to live there again" or something like that
c) specifies that living in London was much closer/right up to the wedding date: 結婚するまえロンドンに住んでいた
Edit: these are very nuanced readings from a native speaker, and based on the fact that there isn't any surrounding context. In a regular conversation they could be more or less interchangeable without being totally misunderstood, though one or the other may be more "correct" or "natural" in a given context.