What Is Inside a Spring-King Mattresses And What Makes Them So Good Tailor-made in the UK, the Spring-King is expertly handcrafted exclusively for MattressNextDay. The mattress collection uses luxury natural fillings like cotton, silk, and cashmere. Providing excellent absorption and airflow benefits these materials give the mattresses resilience, flexibility and durability. Combined with memory foam then nestled on top of the pocket spring core, consistent tension provides an equal level of firmness while contouring to the body with the foam. Offering exceptional support in a range of firmness these mattresses can be with you as early as tomorrow, with free next day delivery.