There are plenty of 25 - 33% hills here in the UK. Here's an example I rode up recently...

http://goo.gl/maps/4ydw5

Honestly, gearing goes to pot on a hill like that. It's so steep that you pull a wheelie unless your right over the front of the bike - and then the road better not be wet or loose or the back wheel will slip. I rode 36 x 32 and it didn't seem to mean anything when on a hill like that - it's just sweat and suffering.

Edit: Also nearby is this short hill - 45 - 50% according to the locals - I rode up it and it really is the limit on what you can keep the bike upright on...

http://goo.gl/maps/JkoMd

A sign along the road proofs little. Height and distance data shows the true inclination.

I think they're measuring it differently than many of us.

ridewithgps.com calculates Roncombe Hill at a 9% max.

http://thecycleway.com/?p=32

Most GPS methods of determining terrain height are not very accurate as their resolution is not high.

I've ridden up the 12% max gradient on Mt Ventoux (I floated up it in 39 x 28) - I'd definitely trust their measurement there and so that's what I'm comparing to. I agree the signs are usually way off the actual gradient on these roads, but seriously 1 in 3 is about right for that Roncombe hill. It is just an absolute wall in front of you. I'm a good hill climber, very light and small and I was right on my limit with 36 x 32. Looking at photos or Google Street View just doesn't give it justice. Descending it you have to lean right back and you feel like you'll go over the handlebars.

Hardknot pass & Wynrose pass in the lake district hit 25 and 33% on the hairpins. I got about 3/4s up the toughest climb on 39-25, before I hit 0 cadence and had to pull a foot out to stop falling off. 28 will be enough I think. Actually the 25%ers are doable with the 25t, but Hardknot requires just one more gear (at least for my legs!)

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Di2x2