Of the 4, I've only attach the frame cover. My stock mudflaps are still fine, so these new ones I'll store as replacements. It wasn't hard to install the cover. I used the metal tweezer of my swiss army knife to insert the leather string through the holes. I was done in 10 minutes.
The black leather frame cover looks nice. When the Brompton is folded, it protects the frame from possible scratches when it comes in contact with the rear tire. It's also easier to carry the bike with the cover because the latter provides a better grip of the frame.
I'm satisfied with my purchase. Will definitely use the mudflaps when the stock ones tear or otherwise get destroyed.