Brompton owners

The largest front bag that fits the flat bars is the ortlieb one. There are other bigger ones but you then need the ugly bars.

That's Brompton's recommendation. It does interfere a little with the cables, but unless you leave the bag open with stuff sticking out of it (I've done that too) it still works and does not obstruct steering.

I have a T bag and the equivalent of the "flat bars" (even worst in fact, as I have slight risers on a M-type stem, meaning the handle bar clamp is even lower than on the S-type) and it is fine. I'm not the only one doing this, other Brompton users I've talk to have said they are happy with that set-up too.

But again it makes sense for Brompton to recommend not too.

(see here: http://www.lfgss.com/post4016853-1250.html
As pointed out before ignore saddle height, I was getting out of the train and had pulled the post out of the frame too much. Here the T bag is filled with cheese and wine as I was travelling back from France after Christmas, and it was riding fine)