Another idea - without the use of electricity! - would be to place sponges in the bowls of water you have. Keep the water level at a height were about half of the sponge is out of the water. The sponge will soak up the water in the bowl, and all of the little air holes in the sponge help the water to evaporate faster.
I agree that it is much easier to keep humidity levels up if you have an enclosed space. I don't know what the weather is like up in London, but just checking the local forecast for Winchester, relative humidity outside is 'low' today at 79%, but the rest of the week its back up around 95% - 96%. Inside my house it is 10-20% lower than the outside ambient RH, but inside my tanks it easily averages 80% or higher.
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