Orange- basically childcare vouchers are a salary sacrifice scheme. So your company take the money directly from your wage before tax/NI is deducted, so you save ££.
When you're on mat leave, then your company have to continue with your contributions even if you earn nothing and can't deduct money from SMP. Free money!!! So they effectively foot the bill as soon as your occupational mat scheme runs out.
The scheme is capped at £200 and odd (can't remember exact figure) so there is a limit. But if you definitely know you want a nursery/registered cm, then worth signing up pre-mat leave.
Anyway, you can let the funds build up during mat leave and start to use them whenever you're ready. Mine sit in a holding account which my employer pay them into.