So if I have decent double glazing, a few windows open a crack, and run a dehumidifier set to 50% why is there condensation?
(I'd say probably because its 80% humidity outside, there are 3 people in the house all day, and even good double glazed glass still gets cold enough to condense moist air. Otherwise why would triple glaze even exist?)
Obviously it's not really possible to answer the question without knowing quite a lot of detail about the house.
But assuming the double glazing is fairly decent then it points to excessive moisture in the air. There are multiple potential causes of that. For example it could be that there is general damp caused by leaking roofs or walls, or it could be that there's insufficient ventilation, for example there is not good enough extract ventilation from the main sources of humidity like the kitchen or bathrooms or areas used for drying clothes.
Ventilation can always be increased - but of course, the more you increase it, the more heat you are losing. Unless you use a heat-exchanger ventilation system, something that is gradually becoming more the norm as houses become more airtight.
Heat-exchanger ventilation systems can be expensive so of course a balance can be struck, where you ventilate (operate extract fans, open windows or use trickle vents) to a point where any condensation is tolerable. If it's a bit of condensation at the bottom of windows for brief periods during the day, and the condensation generally evaporates away later and doesn't cause water to pool up and start causing mould, then it's quite reasonable to decide it doesn't really matter, and you'd rather not increase your heating bills to get rid of something that isn't actually a problem.
But generally, it certainly isn't an inevitability in the UK climate that you will regularly see problematic condensation on double glazed windows.
The reason triple glazing exists is that even if double glazing can generally insulate to a level sufficient to keep the inner pane above the dewpoint in the UK climate, it's still losing heat and triple glazing can provide an even better level of insulation where you can reduce heating costs and increase comfort (especially when you have large areas of glazing).