I don't really think that there is any simple answer to this question: some specific sets were allocated to particular workings, others were special traffic sets which wandered wherever the need took them, but most were rostered in diagrams that could be amazingly complex and required only a particular type of set rather than a specfic set, resulting in sets wandering - the example usually quoted is that given on pages 106 to 108 of SR150, which gives an example of how complex the rolling stock diagrams could be, and one that is hard to better. Stock utilisation varied wildly, while some sets worked intensively around the south east, others only made trips between Waterloo and the West of England on summer Saturdays, travelling out to say Illfracombe one Saturday, spending the week sitting in the carriage sidings there before coming back the next Saturday - some new coaches that the Southern built were used on only about a dozen summer Saturdays days a year.
It is a vast and fascinating subject, about which most of us wished that we understood more...