An important factor limiting the front derailleur operation is of the cage coming close to the seat tube and the derailleur mount itself. When the mounting point is above the band around the seat tube, the cage hits the band. When the point is near the band, the cage can go under the band, closer to the tube. For the K-Series Lightpro you have a proximity window to the seat tube, when you put the mounting point at the very top of the holding finger, as in your photo. However, then the torque bending the finger ends up being the highest then. Also the window narrows and maybe disappears if you get the angle right in the vertical plane for the derailleur - different for Brompton than Dahon. Litepro has a new edition of the mount for K Series, with a stronger finger, but that finger is pretty much free to rotate in the azimuth - no clue here why they mass manufacture flawed designs. In B Series, the mounting tab is presumably secured to the band with one screw. You need to add a second screw and secure both with threadlocker.

As to the specific derailleur, I got a used 105 off Ebay. Ultegra also worked in my memory. Given the challenging geometry and large difference in ring sizes I wanted, I was looking for a long steel cage that could be deformed. To get the angle right, you will need to get the Sram asymmetric washer. You will also need to push the bottom bracket axle out on the drive side, by whatever means, longer axle and/or spacers. For my 3 rings, I use a Suntour derailleur as I found their derailleurs to be slimmer allowing for a better fit against the seat tube.