The taxman.
You pay tax to HMRC but you get tax relief on your pension contributions. Then when you access your pension pot you can get 25% as tax-free cash, 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 2018-19 tax year is £11,850 tax free on income from earnings, investments, property etc, and your pension.