Lifting the Dahlias. Hailing from Mexico and Guatemala originally they don't like Britain's cold and damp winters. Some gardeners in southern England do leave them in the ground covered with a thick mulch. Traditionally they are lifted and stored indoors over winter. 

 In both cases the stems are cut to a few inches above the tubers. If lifted the earth should be removed as much as possible. Some gardeners wash the earth off, some don't. Some leave them upside down for a few days to drain excess water from stems, some don't. Some store them in dry compost over the winter months, some wrap them in newspaper, some simply leave them in boxes or crates (as here).

 The main consideration is to store them indoors in a space with which is dry, not heated but where they will not freeze.  If they get too dry they shrivel, too damp they rot, too cold they die. As long as they make it through to spring they can be replanted for next year's display.