The annuity market is very competitive and the rates differ between companies. You can substantially increase your pension income by purchasing your annuity from the company which pays the most income.
This is called "Exercising the Open Market Option". It costs nothing to take advantage of this option and new rules introduced recently by the FSA means that insurance company must tell you about this option. Check out the best annuity rates.
Source: William Burrows Annuities May 2010