Fleur, just a few ideas off the top of my head for starters. I'm sure others will come up with more.
Atriplex halimus, tough, tolerant of heat and poor soil.
Lantana camara, fast-growing, needs little or no summer water, poisonous berries might be an issue if children involved.
Limoniastrum monopetalum, indifferent to poor soil and lack of water, takes trimming.
Myrtus communis, myrtle takes heat and can be treated much as box is.
Pittosporum tobira, takes trimming but possibly to the detriment of the 'orange blossom' scented flowers.
Rhaphiolepis umbellata or R. indica.