Have you tried Pop In The Bath Bubble Bar? It's the same...so you can layer the scent! I'm not sure there was a perfume of Olive Branch......There definatley was a Shower Smoothie made specially!
There were definatley perfumes of American Cream/Flosty Gritter, Potion/Ruby Red Slippers, Honey I washed The Kids/Ma Bar, Fever/Hollywood/Mirror Mirror and...Snowcake!
So I don't think you've missed out anywhere (I could of course have completley missed it myself)
there actually was an Olive Branch perfume made in the last UK perfume round, try eBay or the swaps forum on the Lush board. don't think they ever made an OB shower smoothie, but they did make an Olive Branch lotion also that was quite nice.