Genuine Brompton replacement chain. The length of chain required depends on your setup:

96 link for 3-speed bikes with 12T/13T/14T rear sprocket and 44T chainring

98 link for 3-speed bikes with 50T chainring, or 6-speed bikes with 44T chainring

100 link for 2- or 6-speed bikes with 50T chainring, or 3-speed bikes with 13T rear sprocket and 54T chainring

102 link for 2- or 6-speed bikes with 54T chainring

You can purchase a chain with a greater number of links and shorten accordingly.