Hi,
The UK taxes income and capital gains on the arising basis, that is, the income or capital gain is taxed in the tax year that it arises. The guidance below advises that any extra payments you get from deferring, could be taxed.
Delay (defer) your State Pension
The rules for arrears of pension and pay, do not apply to the State Pension. As you chose to defer your State Pension the pension is now taxable in the tax year that it arises.
Thank you.