It's a lot less than you think!
I'd put an accurate figure of between 5-8,000 active kitesurfers in the UK. The first issue is you have to define a kitesurfer, are you counting people who have kites, but haven't kited for a year or so, are you counting people who kite 20 times a year or more? I would use the term "active" kitesurfers, ie people who actively do their best to get on the water when they have the time and conditions allow.
If you look at the beaches, thats the best indicator, then at kite sales in the UK, BKSA Membership and magazine readership to give you a balanced view...
On the South Coast, between Brighton and Littlehampton there is a huge concentration of kiters, spots like Lancing will boast in excess of 100 kiters on a good weekend. My own local spot Camber Sands in the South East can have a similar number of kiters on a good day in the summer. BUT and here is the big but, other beaches just aren't as crowded, as someone has mentioned there are not that many kiters in the whole of Cornwall, head North and the story gets even bleaker, just hardcore groups of riders blessed with quiet beaches and none of the scenes we get on the South Coast.
If you go around all the beaches with kiters in the UK on any given day there would be about 3000-4000 kiters out. Factor in that not everyone can get down the beach everyday and my estimate of 5000-8000 is looking healthy.
Kite sales are smaller than you would think, big brands like North, Slingshot, F-One, Best et al don't count sales in the 1000's on kites, it's in the hundreds. Bear in mind some riders buy 3 kites in one hit, and you cans see the industry is actually very small. Of course not many people buy kites every year, so these numbers equate to the estimate I have given already.
BKSA membership is about 4000-5000, they have members across all kite sports, and of course some naughty folks aren't members and either ride without, tut tut, or with someone else's insurance. But I think their membership level is a solid indicator of the size of the sport.
Mag Sales, despite the "print figures" paper magazines give to make themselves sound big, the numbers are actually very small. Singletrack Magazine which is an MTB magazine, bear in mind MTB is massive in the UK, at least 250,000 MTB riders and their magazine is 20 years old with a solid customer base, recently did a blog post and admitted to having a readership of 6,000 a far cry from the claimed 20,000 they were preaching about. And this is true for all niche magazines, with no ABC figures needed to state readership levels they can claim whatever they want, but the reality is the numbers of kite mag sales in the UK is pretty low. Certainly when I worked at a UK kite magazine the readership was around the 2-3000 mark.
Our own mag, http://www.iksurfmag.com
does have accurate readership figures for the UK from Google Analytics, and we have had an average of 4,576 readers in the UK over the last 3 issues of the magazine. You can check the stats if you wish here: http://www.iksurfmag.com/advertise/
So going on the fact that while the mag is good, not every UK kiter reads it every issue, the estimate of 5 - 8,000 active UK kitesurfers I think is pretty accurate and one I always quote in the industry...
Incidentally a while ago Kitesurfing was billed as the fastest growing sport in the world with a total membership base of 250,000, it was in an article in The Kiteboarder and quickly became hard fact. The stat was totally made up by the then editor and was based on no research whatsoever. Most major kite brands and I mean top 5 brands have sales between 10-25,000 kites per year. Again further proof that while growing, this sport is very very small!
Although sometimes it might not seem that way when you are down the beach and it is packed! 32,000 is way off the mark!