The Beds Direct budget mattress is a great entry level budget mattress. It has a soft/medium spring system and is approx 6" deep. Can be used on divan or slatted bases. The mattress is upholstered with fillings on both sides so it can be turned/rotated for longer life.
As with the vast majority of budget mattresses on the market it is designed for use by smaller children or as a temporary guestbed mattress for adults. If you are looking for a mattress suitable for every day adult use or just a better mattress for your child the perhaps consider the Prado mattress as a starting point.
Note: Fabric colour may vary from image