You pay tax to HMRC, but you’ll normally get tax relief on your pension contributions. Then when you access your pension pot you can usually get 25% as tax-free cash (limited to a lifetime maximum of £268,275 unless you hold protection for a higher amount), but from then on, your pension savings will be taxed as income.
Don’t forget, the standard personal allowance you get each year though – which for the current tax year is £12,570 tax free on income from earnings, investments, property etc, and your pension.