Libra as I understand it this isn't actually entirely clear in law yet, it's one of those things that's still being explored.
In terms of when your notice period actually would be, it would be (for example) a month from the date you are notified. It wouldn't be delayed for any reason. It's not a question of them making you take your notice period at a certain time, your notice period automatically starts when you are given notice. The requirement is that they give employees a certain amount of notice of the redundancy, notice in terms of time, and they would be doing that if you were on maternity leave, it would just happen that you were not earning during your notice period.
However you could make the argument that they should pay you in lieu of notice pay that you would have received if you had been at work on sex discrimination grounds because you are disadvantaged because of your maternity leave. Tricky one though. Not something I'd advise taking on unless you are prepared to go through a tribunal, so unless it's huge amounts of money it's probably not worth it.
For what it's worth, if I were advising an employer, I'd probably advise them to pay notice pay to employees on maternity leave to be safe.
Are you satisfied that the redundancy is fair? While on maternity leave you have extra protection, and if there is a suitable alternative role anywhere in the company, it must be allocated to you as a priority.