I'm 6'1 and ride a XL, but would never usually ride the largest size in a range. I tried a L and didn't get on with it, I think because the TT length was 55.5, and that combined with a relaxed ST angle (meaning the saddle has to be a bit further fwd) meant I had v little reach to the bars. If you're in London check out On One's new shop to see them in person.
EDIT: to answer your other questions: you don't need any special headset as the canti hanger is like a separate headset spacer that fits on the steerer. One of the bikes in this thread as a road fork so yes, that would work too.
I'm 6'1 and ride a XL, but would never usually ride the largest size in a range. I tried a L and didn't get on with it, I think because the TT length was 55.5, and that combined with a relaxed ST angle (meaning the saddle has to be a bit further fwd) meant I had v little reach to the bars. If you're in London check out On One's new shop to see them in person.
EDIT: to answer your other questions: you don't need any special headset as the canti hanger is like a separate headset spacer that fits on the steerer. One of the bikes in this thread as a road fork so yes, that would work too.