"Based on the act does that mean that even though I have annual leave and the bank holidays over christmas to take before 1st January if I finish up on the 17th Dec as planned they will consider that the start date of my maternity leave?"
Yes, the state will consider this as the start of your maternity leave. Your employer will also.
If you have a 'use it or lose it' policy in your workplace with regard to annual leave then you will need to use those days before your maternity leave commencement date.
Or, you could ask your employer can you use them at the end of your maternity leave period.