What topper is best for my needs?

Hi 5ft 11 72kgs. My wife and I have a pocket sprung mattress from warren Evans circa £800 4 years old. I get hay fever and it tends to get worse when I go to sleep. I'm sleeping in our spare room on a memory foam mattress which I've found much better on my allergies but it's also much firmer. Can I get a topper for my king bed that is allergy free and makes it feel firmer?
Thanks