We investigate the choice between the relative markers which and that in 8,283 restrictive relative clauses on subject position, with inanimate antecedents, in a written corpus consisting of British and Australian Hansard materials over five sampling years (1901, 1935, 1965, 1995, 2015). Our aim is to determine how processing-related factors and prescriptivism-related factors influence processes of language variation and change across two varieties of English. We analyse how the language-external variables of period and variety (British, Australian) interact with two groups of language-internal variables, namely predictors related to language processing and linguistic predictors associated with prescriptivism. The analysis shows that the relativiser that has been on the rise over the past century. The increase is particularly pronounced in the British Hansard, as which was comparatively frequent in early twentieth-century British material. As to the relative importance of predictors, we find that language-external predictors are the most important in conditioning the variation in relative markers, followed by processing-related constraints. Prescriptivism-related variables tend to generally be less important in this type of variation.