I don't think they look at each job individually, causes issues when merging posts, which was still there ? But they'll take the group of folk doing similar work and have them apply for the similar role positions that remain. Then the rest effectively have no job, so are redundant. You may personally consider that a position that remains is basically what you are doing, but a position has gone and the redundant one could be you.
As for what they can and can't force on you, that will depend. If they go too far then you could claim constructive dismissal, but as to the line that divides a reasonable condition to an unreasonable one, I'll leave that to the experts. However I don't think you can just refuse and claim entitlement to redundancy. If you refuse then you will have chosen to resign.
I'm unsure whether you are hoping to retain a job or get the redundancy payment from your question.