It appears your employer has misunderstood the law in relation to employees on maternity leave. Whilst it is correct that employees do not normally have the right to carry forward untaken holiday into the next holiday year, that is not the case for those on maternity leave. This is because you cannot take holiday whilst also being on maternity leave. Preventing you from carrying it forward would be a detriment you are suffering only because of maternity, and therefore would be discriminatory and unlawful.
I recommend you ask your employer to review this again. Explain that your normal full holiday entitlement continues to accrue throughout your maternity leave period, and that you cannot take holiday and maternity leave at the same time. This means that, in addition to any untaken holiday prior to the maternity leave period, you are also entitled to the benefit of the full holiday entitlement for the duration of your maternity leave. The law does not require you to use that holiday within a specific period of time after the maternity leave ends. Your employer might ask if they can pay you in lieu of the holiday accrued, but they cannot insist on this as their obligation is to allow you to take paid holiday.
Practically, you could agree to take some or all of the accrued holiday after the end of your maternity leave or in the next holiday year once you have returned from maternity leave, and that is what we typically see. If they refuse to allow you to take your accrued but untaken holiday and/or unilaterally treat some of your maternity leave period as holiday instead, I suggest you raise a formal grievance and, if that is unsuccessful, speak to ACAS under early conciliation in relation to your claims for discrimination for taking maternity leave and failure to allow you to take your accrued holiday.
*Maria Hoeritzauer is a Partner at Crossland Employment Solicitors in Abingdon.