It is much less generous than I seem to remember it used to be, as it is capped to a maximum wage of £380pw which is used in the calculation.
"The calculation for your statutory redundancy pay is based on:
* how long you have been continuously employed
* your age
* your weekly pay, up to a certain limit (£380 current maximum)
You will get:
* 0.5 week’s pay for each full year of service where your age was under 22
* 1 week’s pay for each full year of service where your age was 22 or above, but under 41
* 1.5 week’s pay for each full year of service where your age was 41 or above"
Full details here: http://www.direct.gov.uk/en/Employment/RedundancyAndLeavingYourJob/Redundancy/DG_174330
As someone else commented some employers used to voluntarily pay more than this, often to get people to go voluntarily, but that seems to be going out of fashion.