COVID is also growing among the vaccinated because the number of people in the UK who have had both doses is continuing to rise. At the time of writing, 88% of UK adults have had a first dose and 69% a second. As more and more of the population is vaccinated, the relative proportion of those with COVID who have had both jabs will rise.
If you imagine a hypothetical scenario in which 100% of the population is double vaccinated, then 100% of people with COVID, and in hospital with COVID, will also have had both jabs. As with deaths, this doesn’t mean the vaccine isn’t working. It just means the vaccine rollout is going very well.