No, you can choose to move all or part of your pension fund into drawdown. Moving only part of your pension into drawdown is known as partial drawdown and may be more tax efficient. Each time you move a part of your fund into drawdown you can usually take up to 25% tax-free cash from it. Income taken from the rest will be subject to Income Tax at your marginal rate.
Each time you want to move funds into drawdown you need to complete the relevant SIPP benefit form.