johnd, it's interesting to me you your British customer base seems to not care much about relative humidity. Here in the mid-Atlantic USA, sweltering humidity can make RH and Dew Point data very important to folks.
Don't forget that the mid-UK latitude is around 55N. It's a predominantly damp climate and RH much below 40-50% is uncommon, while Dew Point much above 20C is also pretty uncommon, though that doesn't stop people complaining about discomfort if the DP rises much above 15-16C. For instance, today is a pretty average, somewhat overcast, July day in southern England - temperature at 1500Z is 21C; DP around 15C (though DP was a degree or two higher earlier).
And, just to be clear, I'm not suggesting that RH/DP is of zero importance here, it's more that wind, rain & temperature seem to be of much greater priority to users than RH or, within limits, pressure.
What might happen in the years to come about RH perception is a different matter of course, though it's not impossible that if the North Atlantic Drift (as distinct from the Gulf Stream) should switch off - as some speculate might happen - then the UK could conceivably even cool off a little relatively, at least for a limited period until overall warming overcomes it again.