As a consumer/SME company scheme, it really will depend on the condition itself. How long since you were diagnosed? Do you still have treatments or check-ups relating to your pre-existing condition?
If you already have a policy in place, is the condition already declared as an exclusion?
There is not an easy answer to this question however, in some cases, it is possible to obtain cover for pre-existing conditions. The underwriting type you choose will also play a big part in the answer.
If you are joining a corporate company paid scheme that has Medical History Disregarded (MHD) underwriting then you will have cover for conditions diagnosed before the policy start date. The chronic condition terms and conditions will still apply.