According to this, tax and NI will be deducted -
https://www.gov.uk/maternity-pay-leave/pay
//Statutory Maternity Pay (SMP) is paid for up to 39 weeks. You get:
90% of your average weekly earnings (before tax) for the first 6 weeks
£139.58 or 90% of your average weekly earnings (whichever is lower) for the next 33 weeks
SMP is paid in the same way as your wages (eg monthly or weekly). Tax and National Insurance will be deducted.//
There is also a calculator on there which may help with budgeting.