If your holiday year runs January - December and you're due at the start of January/know you'll be off for the full year and will accrue a full years leave. Can you take the full 28 days of that year at the start of maternity? Or are you expected to keep bank holidays in the months the fall/get paid more for them on that particular month if by that point your down to SMP?
Hope that makes sense
At my work you had to start mat leave the Sunday before your due date. You could take holiday before that or use unused holiday at the end of leave even if you had to carry it over or get paid out.
Check with your hr department
When you take it is up for negotiation between you and your employer. You can take it all before, you can take it all after or you can take some before some after. Discuss with your employer what you want to do.