Where an employee who is about to go on maternity leave applies for an internal vacancy, must the employer consider them for the position?

Yes. If a suitably qualified employee who is about to go on maternity leave wishes to apply for a vacancy, they are entitled to be considered for the position even though they may not be able to start for a year. If the reason they were not considered for the post was on the grounds of their imminent maternity leave, this would amount to an act of pregnancy and maternity discrimination.