Mattress for 4yr old with eczema, asthma and a dust mite allergy

Hello,I came across your website by accident and I am really hoping you can help as I am so very confused now about what kind of mattress is suitable for my child. My son is moving out of his toddler bed, into a single bed. Since birth I have been using natural mattresses from Naturalmat, though the mattress on his toddler bed has been a John Lewis foam cotbed mattress as I wasn't able to afford a naturalmat at the time. He hasn't been on that mattress for very long. Now I would like to get a mattress which will support his growing body for years to come, and which is suitable for a child allergic to dustmites, but also breathable and keep him cool to keep his eczema at bay. I think i have read that springs are more suitable for a growing child, but latex/foam is more suitable for allergies. I am so very confused and was considering either the "Teen Spring" or the "Natural Teen" Naturalmat mattresses . In trying to do more research I found your site and noticed you do a very similar cot mattress. I am really hoping you can help me find the right mattress! I am will to spend up to £700 for the right mattress. However in order for me to spend that much money, I need to know it is the best choice for my child. I am very keen on getting a mattress which uses natural materials and a bonus would be a cotton encasing (not sure what the technical term is!)I would really appreciate any advice you can provide!Many thanks in advance