I have a bad humidity/condensation problem in my house, very similar to what the OP describes.
Had an expert come and check it out. Conslusion was it was definitely all just due to ‘living conditions’ – i.e. condensation/damp generated by ourselves as opposed to damp ingress from externally.
Suggested solution was fitting a ventilator type thing in the loft that would blow dry air from the loft space into the upstairs hallway, thus creating a slight positive pressure and forcing the damp air in the living space to circulate more and be pushed out through vents/open windows.
£500 fitted apparently.
I might try a dehumidifier for now as I can’t afford the £500.
Plus, I figured that blowing freezing cold air from the loft into the house is going to wreak havoc with heating bills.