Riding with a saddle even slightly too low is no fun. You can win around 15mm of extra height by moving the saddle-clamp up the seat pillar itself, and this may be sufficient. If not, there are two Brompton options which give you more height: an extended seatpost and a telescopic seatpost.
For the maximum height, draw the seat pillar up until it reaches a positive stop. At the minimum height the seat pillar should not protrude below the main frame. The minimum saddle height is suitable for a 26” inside leg measurement, making the bikes suitable (for the most part) for adults only.
In order to set the correct saddle height each time - check out the Saddle Height Insert