Yes you have to treat people equally at work! You can't favour one person over the other.
Nope. To echo previous posters, fair treatment is not the same as equal treatment.
The change of rules around holiday carryover due to covid doesn't mean that employers have to let people carry over more holiday for the next two years. It's up to the employer.
Essentially, your employer is making you stick to the normal carryover rules - not putting you in any worse position than normal, just choosing not to offer you additional carryover. To make sure you get opportunity to take all your leave this year, they're enforcing some leave in October before you return so that it works best for business requirements. That's perfectly reasonable.
There are some other people at your organisation who they have chosen to let carry forward more holiday because they are in a different situation to you, having worked all year and not had the same chance to take annual leave. Again, this is reasonable.
Nothing about this is discriminatory.
I understand that furlough hasn't been a happy time for many and I'm sorry that this has been a hard year, but it hasn't been a walk in the park for employers either. If you were struggling on the reduced income you could have asked to use some holiday in the last few months to top up your earnings and at least you'll get full pay for the period in October you're on annual leave so there is some positive to this.
Also, on this:
they can't use your grievances against you.
They can't explicitly use a grievance as a reason to select for redundancy, but in reality it's not hard to design a scoring matrix that keeps the people you want and makes sure that the pain in the bum team members end up at the bottom of the list.