If income is sourced in the UK, as is the case with a UK pension, then even non-UK residents are liable to pay UK income tax on those earnings in the same way as UK residents. Your UK pension, counted as UK source income, will therefore be taxed at your marginal rate. Tax is applied to the 75% that is not included in your tax free sum.
A Double Tax Agreement (DTA) between the country in which you are resident and the UK can mean that you are exempt from UK tax. If the DTA exists between the country in which your pension is held and the country in which you are resident, than you can choose, instead, not to pay tax where your pension is held in favour of being taxed where you are resident. International pension transfers is a potentially complex business and we would always recommend bespoke independent financial advice to expats considering this option.