I think its more important to work having children around your biology than around finances. Like someone else said, even when things are going well financially- you still cannot predict what tomorrow will bring (ie- a recession!), however you CAN predict your biological age and that is something that will not change.
You will find that babies cost very little in the first year anyway. The baby bonus pays for the pram, cot etc. If you host a baby shower its unlikely you will need to buy many clothes, toys etc (at least for the first 6 months), and if you breastfeed, your baby will cost nothing to feed for the first 6 months of life.
I think the suggestion to receive half of your maternity pay over 24 weeks instead of the full pay for 12 weeks is a brilliant idea. I don't want to put you off going back to work early (that is entirely up to you), but in my experience, it takes a good month for your body to recover from the birth itself (you bleed for quite an extensive period), and most women are still feeding their babies a couple of times a night at 12 weeks and this can leave you feeling rather exhausted.
I think that at 24 weeks you would feel much more prepared physically and emotionally to handle going back to work - but that is nothing to do with guilt or anything - just purely from a health and re-cooperation perspective. Mothers need to take as much care of themselves in those first few months as they do their babies!!
Do you have a mother/mother-in-law that will be able to help out with bub when you go back to work a few days a week? This will save you alot of money on day-care expenses, plus the assurance that your child will be getting one-on-one attention.

Goodluck with your decision and don't feel bad for whatever you decide.

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