Tbh I really dont knw where to start....so many so its confusing
Wouldn't buy an open coil mattress (Miracoil) unless there was no alternative. For £350 you can definitely buy pocket sprung.
Same...but I only need the mattress
Miracoil is an obsolete spring system for mattresses. Save your money and look for Mirapocket/pocket spring.
Did do a quick check myself and couldnt see anything obvious this looks a great deal for king size