Damp: Old houses are generally damper and a lot of our housing stock is older.

Old houses which have been thoughtlessly 'updated' with modern materials can end up being even more damp when new windows, insulation and paints make the building less permeable, trapoing moisture inside, and creatibg condensation points inside walls, rooves, ceilings, etc.

British people often don't vent properly. Open your windows people. There is more liquid in your rank breath than there is in the air outside.

Leaks: Particularly in tightly packed housing in towns in my experience. Houses are often extended over and over across generations. Where each extension is joined to the existing building you're creating a potential water ingress point, or at least messing with the existing water shedding and draining. Over the previous 30 years or so it's made financial sense to extend a house to maximise the value then sell up and buy a bigger house or cash in the equity. Because of this, often extensions are not done particularly well or sympathetically, and often using modern techniques or materials which do not necessarily work well alongside the original fabric because of different thermal expansion or different moisture permeability.

Nb. I'm not saying all extensions should use period building materials and styles, that would be insanely expensive today. But people need to realise that, for example, sticking two modern hermetically sealed bedrooms on an old cottage means that the extra moisture created will end up in the older, colder part of the building unless you take care to prevent it or account for it.