Best I could find
www.maternityaction.org.uk/advice-2/mums-dads-scenarios/pregnant/discrimination-during-maternity-leave-and-on-return-to-work/
"I will not be able to take all of the annual leave I will accrue during my maternity leave because my maternity leave ends shortly before the end of the annual leave year at my work.
It is very important to discuss with your employer when you will take your annual leave. Ideally this should be done well before your maternity leave. If you have accrued a lot of annual leave by the end of your maternity leave you will need to discuss with your employer when you can take it. You could give notice to end your maternity leave early, if you wish, and take paid annual leave immediately afterwards. You must give at least 8 weeks’ notice to end your maternity leave early and you must agree your annual leave with your employer in the usual way.
Your employer can refuse annual leave where it coincides with a busy time of year or a lot of other employees are taking leave at the same time but they should not refuse to allow you to take annual leave because you have been absent on maternity leave, as that may be maternity discrimination.
If you have returned to work early you and/or your partner may be able to take some shared parental leave within the first year from your baby’s birth. For more information, see Shared parental leave.
If you are unable to take all your annual leave in the leave year because of absence on maternity leave, shared parental leave or other types of parental leave, the government advises that employers should allow you to carry forward up to 28 days statutory leave to the next leave year. The government intends to amend the Working Time Regulations to allow 28 days to be carried forward if a parent cannot take it, see www.gov.uk/holiday-entitlement-rights/calculate-leave-entitlement.
If your employer offers more than the statutory minimum of 28 days annual leave, it is up to your employer how much of your contractual annual leave you can carry forward so you should check your contract or talk to your employer.
If you are not allowed to carry forward your annual leave you should seek legal advice. See below for where to go for more help."
Another article I read said the reason govt recommend this is because of the decisions of the European Court of Justice. Not just a whim.