Go to the Citizens Advice bureau and ask what benefits you can claim in various cirumstances and how much they are worth.
Unless you are married, yes, her pension is worth something even if it will be small. If you ever split up (I note you do not say you are married or getting married before you move in so she is vunerable) she could be left penniless in her old age as it is very possible she will not be able to claim your pension.
How much is the father of the child contributing and could he be forced to pay towards its upkeep?
Would you be better off financially living in seperate buildings and just getting togeather a lot? You might be able to claim more benefits and also if you have more cash, there will be less stress and rows and a better financial quality of life for the child.
Saddly, if you have more kids pretty sharpish you can claim more and more on the state and get better and better living conditions. I know someone on thier 3rd upsize of car (to extra large people carrier) as they are now on thier 6th child and only 1 parent earns on a low wage. His wife is keen to have more but he has said no, he does not want the responsiblity of bringing up more children!