The key thing here is really company policy around annual leave. Mat leave has to start from the date you give birth at the latest, because it is legally required that a woman who has given birth has at least two weeks off work after the birth (four weeks if she works in certain environments, like a factory, IIRC!). It’s built in protection to ensure all women in emplyment have time off immediately after giving birth. This is why you can’t take annual leave up front and over the period in which you give birth, and instead have to either take it prior or after.
The prior or after however depends on your company policy. For example in my workplace you are encouraged to take any currently accrued annual leave prior to mat leave, because otherwise you could end up accruing an unsustainable amount. Let’s say you take a year of mat leave, which is common where I work, you would have a whole year’s entitlement accrued during your mat leave plus whatever you didn’t take before you went on leave; and you might need to ideally use it all up before the end of the annual leave year on your return, which might be hard for the business to sustain, etc.
I’ve also felt it would be great if you could shorten your leave but take all of the SMP. Eg SMP pays just shy of £630/month, let's say you earn £1200/month (for example), you might be able afford, say, five months of leave because it would be about the same as your full time salary. Essentially having access to an SMP ‘pot’ of £4790 plus whatever your 90% is for the first six weeks, but you determine the timescale (within the permitted 52 weeks of leave, so you could drag it out longer than 39 weeks or get it paid shorter than that).
Perhaps it would be a nightmare to administer but I don’t see how. Large companies deduct the SMP costs from their NI contributions; small companies can apply for an advance in full for the cost of the SMP. They just recoup the cost as needed, it doesn’t go beyond the SMP permitted.
I’ve thought about this a lot
(it would make my mat leave planning for later this year much easier if I could do this!). Sorry, bit of a tangent there.