These boots by Buffalo come quite close to what I am looking for.
This Schu pair might also work, but sadly my size is not available.
This pair by Pedro Miralles is more of a sandal with the open sides, but they are still the basic shape I am looking for.
You may think that boots are not the most practical item of footwear for the summer, but shoe boots will work anytime of the year.
The above styles are all available at Spartoo, where UK shipping is free all year round.