If your pension is a money purchase scheme (accruing a pot of money for you), then you could take 25% tax free; you could also take the complete pot of money, but you would pay tax on the remaining 75% (this could result in you being tax at 40%).
But to get that 25% tax free, you need to end paying in to the pension (and make other arrangements for the remaining 75%). If your employer would otherwise continue to contribute to the pension, then it would almost certainly be a bad decision to end the pension (and the free money from your employer).
The same principals apply if the pension is a define benefit scheme, normally the pension provider will allow you to take a lump sum, reducing your pension pay out by 25% as a result.
Bear in mind that the pension provider decides on a notional figure as to how much money they need to have to pay out your pension – and then gives you 25% of that.
But again, you would then need to start drawing the pension at 55 (with the resultant reduction in payment, given that you will be receiving the pension for longer).
As TonyV says, you would be well advised to seek advice from a financial adviser nearer the time.