There are two main reasons premiums go up each year:
The cost of medical treatment and tests increases all the time: The medical world doesn’t stand still. Scientists develop new treatments and tests, create new medicines, and find new solutions for conditions that previously couldn’t be treated well.
Also, as new equipment and procedures become more available, doctors tend to use them more. In 1996, a cancer patient had on average 1 scan during their treatment. In 2016 they had 9 scans.
All of this is great news for patients, because doctors can treat more conditions more successfully. But it does increase what we must pay out for claims, and therefore your premiums.
Everyone tends to need more medical care as they get older:  Unfortunately, as we get older, we all tend to have more health issues and need more medical care, which is why high-quality health insurance becomes even more important. However, this does mean that all health insurers tend to increase premiums as people get older.
It’s also worth remembering that moving to other providers may cause your existing conditions to be excluded from cover.