Every financial year (April to March) you pay tax and national insurance on any amount of income above your personal allowance. For this financial year this is £5435. SMP is payable for 6 weeks at 90% of your normal wage and then 31 weeks at £117.18 and if that was your whole income from April to March (working on the assumption you currently earn £200 a week before tax,etc) that would make an income of £4712.58 so you wouldn't have to pay tax an NI. Employers usually average the tax over the year so it may be they take more tax than they should because of this change in earnings in which case you simply claim it back at the end of the tax year when you get your P60.
Your baby is due the end of October so assuming you started maternity leave at the beginning of September you would have received 5 months pay before starting maternity leave - £4000. There are then 30 weeks until the end of the financial year - 6 at £180 and 22 at £117.18. So your income would be £7658. You would have to pay tax and ni on £2223 of this so at the end of the financial year you would have paid in total £444.60 in tax and £244.50 in national insurance contributions. If you paid more you could claim it back. If you go back to work before the end of the financial year your would earn £57 a week more after tax an ni than if you stayed on SMP.
Your post makes it sound like you are considering not going back to work at all because of SMP. You only get it for 39 weeks, though you can take maternity leave for 52 weeks, the last 13 being unpaid. If you choose to stay home I don't think you are entitled to much in the way of benefits as it's a choice. Not much of one if all your salary goes on childcare though! You might be entitled to working tax credit then.
