As standard, the Bromptons come with a plastic saddle - reasonably comfortable, but nothing special ;-) Optionally, you can have:

Vitesse saddle: This is a lighter weight saddle with a tubular frame and a very comfortable top - made by Fizik. See above.

Brooks saddle: The traditional all-leather British saddle, and very comfortable once it beds in. Available with steel or lightweight titanium rails. See below.