Originally Posted by
Ozonation
The Brompton does fill the B&W case fairly well, so you're probably fine with it as is. When I got the B&W case though, I skeptical about the rather thin padding it came with, so for a cheap investment of $30 of foam, I figured that cost was worth it to protect an $1800 bike.
The stock foam in the B&W case is almost pointless, in my opinion - it's so soft and light that it's immediately completely crushed or penetrated by any protrusions. Maybe it gives some scratch protection.
Do you also have a sheet of foam under/over the bike, i.e. where the B&W case's stock foam was? One thing I do find is that the fit is tightest in the depth axis. The case squeeze the bars / stem, and one of the cables (I forget which) is really squeezed against the fork. I have helicopter tape there to protect it.
That said, the bike has been completely unharmed in a dozen trips or so.