The Brooks saddles are nice - I have enjoyed them in the past - but they are very heavy, which may or may not matter to you, and they are not cheap but perhaps, if you are buying a Brompton, that is beside the point..

The standard Brompton saddle is actually quite good. I have done 60ish mile days a couple of times and it worked fine. It is much lighter than the Brooks but doesn't say "I am leather and have copper rivets". It seems that many buy saddles based on appearance, which is folly in my opinion. But if your time in the saddle is short, it really doesn't much matter what you ride, so maybe appearance would be important. And yes, for many, they are very comfortable.