It is true that private healthcare doesn't cover chronic illness, only acute or relatively acute. I have Crohn's and was covered for diagnosis and initial treatment.
To be honest the NHS has been so good with the management of it I wouldn't have it any other way. I think that private healthcare does do it's job well, at the beginning of an illness or condition it provides for quick diagnosis without having to hang around for 6 months for an NHS appointment. Once the condition is diagnosed and treatment stabilised then they hand over to the NHS who are there for that sort of care.
If they continued to pay for lifetime treatment of chronic illnesses, could you imagine what the premiums would be? We would end up in a situation like they have in the US where people are stuck in jobs they hate because they don't want to give up their substantial healthcare benefits.