Very rare to get much over 8-10%, and often only 5%, but goes on forever. Many of the big climbs are 20km+
I would love to climb stuff like that on fixed. Long shallow climbs are just ace - I loved stuff like Le Roc from Brest and totally crushed it on fixed (though that is only 3 or so % and much shorter than something like tourmalet at 20km of 7% average).
But I hated descending for long times, just so hard on your body. I could almost concede that single speed is a good idea for hilly terrain........
It is so different to anything you will have ridden here, especially Wessex. Nothing short and sharp, all very long and shallow. As I said, you basically spend the day putting in hour plus hard efforts, followed by 20-30 mins of descending (on gears), repeat, repeat, repeat.
Anyway, the real true joy of the mountains is sweeping down the other side at 80-90kph!