Its not discriminatory, its just an unsustainable HR policy. In the situation you describe, who gets the best holidays is allocated on the basis of who is the keenest to book them, or who gets earlier to the office. On the other hand, your wife working in the NHS-as does my gf so I feel for you, shouldn’t affect your companies policy.
Think strategically as what an ideal outcome looks like, and then assess what is actually achievable, only after speaking to HR.
edit: just noticed you said 2021, booking up all holidays that way when you know the rest of the team also has those needs is just being a right ****. Also you shouldn’t have to cover for someones maternity, the extra workload involve isn’t your problem.