Well, I think the most of us owns one.....but may be not from the UK, so the running cost change from country to country..not sure about electricity cost there.
Dryers choiche:
For personal experience I can say european models generally changes like night to day from american ones... simply incomparable... european are kinda "toys"...
Over here in Italy because of high cost of electricity (the hghest of europe) dryers are not that common, but gas models sold from speacial dealers along with agitator washers are spreading always more...the cost of these machines is prohibitive for the most though...
Are also sold british gas models now, that generally have lower costs, but they're still averagely about 700-800 euros vs. the 1200- 1800 or more of US machines..
So since you're in UK and gas models of White Knight are "cheap" over there, about 350£ I suggest you go for it if you have a gas hook-up available... not sure about gas costs in UK but generally gas is cheaper than electricity...and they're usually faster.
I never watch energy labels....I think they're BS..... most dryers rated A, A+ yes they have lower consumption of Kwh, but usually they takes longer than others rated B or C... so that the consumption they have at the end is the same or sometimes more.
Not to mention condenser models, they generally takes longer than vented ones.... drying is different, laundry does not come out well as if dried with a vented one, but of course good (and only) choiche for those that can't vent outside.
So if you've the possibility I suggest you a gas model, or if not possible a vented dryer... Vintage, even better.
Leave alone newer stuff...
Even cheaper:
http://www.greenandeasy.co.uk/white-knight-eco43a-gas-tumble-dryer-in-white.html