Most Victorian and Edwardian houses were built with a damp-proof course (DPC) of two or three layers of slate. After 100 years this is often not 100% effective but remember that houses many centuries old have survived with no DPC.
Some surveyors, builders and damp companies recommend a chemical, injected DPC, with replastering, but injected DPCs are not as effective as the companies selling them claim. They will never cure damp problems completely and are far less effective when injected into already damp walls than if they are injected into dry ones. The chemical cannot displace the moisture and will therefore form an incomplete barrier. Ideally the injection should also be into the mortar beds and not into the bricks as many companies tend to do. It is therefore unlikely that you will ever get a long-term, perfectly dry wall.
Instead, take our other advice on stopping water reaching the wall and maximising evaporation.
Concentrate on looking for and eliminating the causes of damp. For instance, just making a small gap between a path/patio and the wall can have a major damp reducing effect.
Cavity walls began to be used in the late Victorian period. Again, these usually incorporate a slate DPC but this can be bridged inside; surplus and decaying mortar from the wall may drop to the bottom of the cavity. If there is a sufficient quantity, this debris may build up to a point above the DPC, bridging the inner and outer skins. The only solution is to cut out bricks at intervals, below the DPC, and rake out the debris. This can also occur above window and door openings. This problem has appeared in houses built as late as the 1930s, particularly where lime has been incorporated in the mortar.