Use a saddle of your choice on your Brompton.
The Pentaclip attaches your saddle to the seat post and fits standard sized saddle rails, so you can use a saddle of your choice on your Brompton. Adjustable to allow fine tuning of bicycle fit, the clip has a high and a low position for saddle placement, allowing you to a get a few more centimeters of height on any seat post length. The angle of the clip is also easily adjustable for perfect saddle pitch. Tools required:
- 5mm allen key
Full technical information here: https://trade.brompton.com/Uploads/QPart/QPart-Datasheets/ds-pentaclip.pdf