Well, I'm going to go against the mumsnet opinion and say that it is not on to have children that you know - when trying for them - you cannot provide for.
If you have children, and find your circumstances change - fair enough. If you are in a bad position and find yourself accidentally pregnant - fair enough, no contraception is 100% effective
But if you cannot provide for your children and you rely on state support (which is a lifeline for people in need -I know!) then is not on to carry on having child after child. It just isn't. It isn't fair on your existing children, it isn't fair on the new child, forget about it not being a fair burden on an already collapsing welfare system!
And people in 3rd world countries have big families when they have no money, yes, but those children are also relied upon to care for the parents in their old age, to feed and look after them! Plus, sad to say, having lots of children is no guarantee that they will survive so it is a totally different thing, different life, and not comparable.
I think that the system should change. However many children you have + 1 (or multiple birth!) should result in a £ increase. After that it should be food stamps and clothing vouchers etc etc.
Because at the end of the day, we are each responsible for our own families. State support is supposed to be a short term lifeline while you get back on your feet, or a lifeline for people who are unable to work. It should not be used for a growing family for their entire lives and it's really sad that there are people who view it thus and they should be helped and supported to see that there can be a different future for them.