The shirts are likely made by Prada but using Holliday & Brown designs actually archived from the '60s/70s. Prada has been having a field day with the retro mod British look. At least they are authentic designs, though.
No, the shirts were actually made by Holliday and Brown for Prada (Prada cuts; Holliday & Brown materials, patterns and manufacture.) Â These types of cross-over collaborations are becoming increasingly common.
Fred Perry and Comme Des Garcons comes to mind.