We have a product in Australia called "Vegemite" which is also a yeast extract product. The labels on the jars are a very thin plastic and are easy to remove if you soak the jar in some hot water. Once you get the label off and let it dry, you can attach it to most surfaces, like your uke case. It will also attach to most uke finishes, but its hard to remove from a wood surface without damaging the finish.
You use stuff called acrylic binder, first put a coat on the place where the motif is going and let it dry. Then saturate the motif in the binder (top and bottom) and paste it on. Finish with a neat final coat. If you like to be really neat you can mark the area with some masking tape so there is a neat border around the motif.
Two motifs are on the photo which should have attached itself, one is off a Vegemite jar and the other is off a Margaret River Dairy Company packet (thin cardboard).
Perhaps you can do the same with Marmite labels, although if you have ever had some Vegemite in hot toast with real butter, you would never think of Marmite again.