As others have asked - How old are you? If you aren't 55 yet, forget it till you are.
You also refer to a 'pot'. I assume that means it's a real pot of money/investments - called a Defined Contribution Pension. Private Pensions, SIPPs and Stakeholder are the 3 main types of DC Pension. If it's not a DC pension , and it's a Defined Benefit/Final Salary scheme then you'll need to refer to the rules of the scheme to see when/how you can access it. Public sector schemes are usually DB not DC - Things like Police, Fire, NHS, Local Govt, Teaching etc are all DB not DC.
So, assuming it really is a DC scheme, and you're over 55, then you can take (up to) 25% of the pot tax free. You can also take none/some/all of the remaining 75% at any time, but...
1) You will pay tax on any/all of the 75% you take. It is counted as income, so you pay income tax at whatever rate applies.
2) If you take ANY money from the 75% you are limited to what you can continue to pay into any future pension to £4000 p/a.
3) You don't get to take a second 25% of the remaining 75% tax free next tax year. You only get one 25% bite of the cherry.
If you don't touch the remaining 75% then you can continue to put more than £4K p/a into your other pensions.