Sorry for my lack of details. So my brompton is a M type.

The handlebar has a "comfortable" shape but I swap it with a low rise to have a more "sporty" position.


Brompton 5 by Teddy.UK, on Flickr

I was thinking to use a bullhorn to have also different positions while riding. I need a Bullhorn with a little bit of down rise (2-3 cms) because of the folding otherwise the "horns" will it the front wheel.
Like these:
or http://www.ebay.it/itm/221067930986?...84.m1423.l2649

So, while having a 23.8 size diameter for the brake levers is not a problem, I don't know if it is possible to find clamps for the brompton gear shifters. They are 22.2 mm.