i'm happy to stand to be corrected, but I am in UK (albeit, in the North West).
I have played acoustics for 20 years and have NEVER met anyone who has felt the need to humidify their instruments. A mate did once refer to a guy who did, but he had bought a vintage Martin for about £4000...
That said - heating habits can ruin instruments in any environment. Whilst I REALLY dont think UK climate should be a cause for concern (hey, this isnt Nevada!) - central heating needs to be taken into account.
I always keep my instruments when not being played in spare room - whilst spare room has radiator, it is turned off (or very very low) and door shut. That room therefore keeps a typical UK climate going on (ie Humid in summer, and cold, but wet and damp in winter). My instruments are fine.
If you cannot isolate them, and you find yourself (particularly in winter, not summer) having to keep them in central heated, warm rooms, you may want to either consider another storage room, or humidifying.
Hope that helps- I think my jist is this- if you have another room, and you are in UK - dont worry too much!