Do I have to take the whole 25% in one go?

No, you can take 10% to begin with and more later on. The 25% is calculated at the time of requesting to withdraw monies. As such, it often pays off to invest your pension pot and let it grow. For example.£100,000 pension pot = £25,000 tax free. 5 years later,  £150,000 pension pot = £37,500.