This depends on the kind of money purchase pension you have.
Of course if you take a lump sum you will have a smaller amount left with which to buy an annuity and thus get a smaller pension, but you can use your lump sum to buy what is called a 'purchased life annuity' rather than a pensions annuity. This has some tax advantages.
The FSA has a downloadable guide with more information on this whole area here.
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