I'm always confused about this because the UK government actually recommends that method

Why is the chickenpox vaccination not part of the UK's routine childhood immunisation schedule?

There's a worry that introducing chickenpox vaccination for all children could increase the risk of chickenpox and shingles in adults.

If a childhood chickenpox vaccination programme was introduced, people would not catch chickenpox as children because the infection would no longer circulate in areas where the majority of children had been vaccinated.

This would leave unvaccinated children susceptible to contracting chickenpox as adults, when they're more likely to develop a more severe infection or a secondary complication, or in pregnancy, when there's a risk of the infection harming the baby.

We could also see a significant increase in cases of shingles in adults.

When people get chickenpox, the virus remains in the body. This can then reactivate at a later date and cause shingles.

Being exposed to chickenpox as an adult (for example, through contact with infected children) boosts your immunity to shingles.

If you vaccinate children against chickenpox, you lose this natural boosting, so immunity in adults will drop and more shingles cases will occur.

https://www.nhs.uk/conditions/vaccinations/chickenpox-vaccine-questions-answers/