I have not used the dehumidifier yet, it can hold like 20 or so gallons but even when it rains it drops the relative humidity enough that its comfortable. Where I'm at (South Carolina) its usually pretty humid so I'm sure you won't have to worry about it (depending on where you're at in the UK). I didn't know all that about evaporative coolers, I just didn't get one bc I needed a dehumidifier as well so got one that could do both.
My AC is also very close to my chin, and it does not bother him one bit bc its pretty quiet for an AC.