In Australia, state libraries are a great source of family history resources. Past electoral rolls are often found there, and they have certainly helped me to trace the steps of my husband's relatives since the time they became naturalised.
Tracing the journeys of my own ancestors in Britain has mainly been achieved through census records, but next year I may also have easier access to the electoral rolls there:
findmypast.co.uk and British Library digital project
Perhaps I will be able to find out, through the electoral records, which of my Belgian ancestors became naturalised British subjects, and which ones did not.