If a soft or medium mattress isn't firm enough for you, then this mattress might be a good choice. If you have allergies, you'd be interested to know that this mattress is allergy friendly.
From Debenhams
Essentials Memory Foam Mattress - White - Double
House of Fraser
Aspire Aspire Essentials Memory Foam Mattress - White
This product is also available at 1 unverified store.