Performance related bonus reduced after taking redundancy

Hi all,
I'd really appreciate a steer in the right direction with this one as to whether I have a "case" and if so how to pursue it.

Summarised problem:

Left former employer via voluntary redundancy, performance related bonus was less than anticipated due to underperformance (normally a high performer), feel like company using VR leavers to hit the quotas of underperforming employees.

Bit more detail problem:

I recently took voluntary redundancy (VR), at my former employer, end of March to be precise. A few months before leaving I completed a performance review.
I received a letter from my previous employer the other day to confirm that the company would pay out the performance related bonus for this year, this included VR leavers as the leave date aligned with the financial year.
However, my performance was rated below average. (2 out of 5 for simplicity). 2 receiving 60% of bonus, 3 - 100%, 4 - 120%, 5 - 150%, 1 - 0%.
In my 6 years with the company I had only ever scored 3s and more often 4s. My mid year review during my final year with the company I scored a 3 and at no point was a conversation had with me about my performance being below par. We were always told that your end of year score shouldn't be a surprise, hence my surprise when I received this scoring. In the VR info it stated that there would be no internal appeal process to the performance rating.

As a bit of background, the company's performance grading works like a normal distribution whereby a percentage of employees need to be in the '2' region similar for the 3s and 4s etc.

A long story short, but I feel as if I have been unfairly dropped in the underperforming category as an easy target as I would no longer be at the company and hence unable to appeal. From speaking to a few old colleague I am aware of a few more instances of this happening.
It should be noted that the bonus amount is not massive and to be honest, it’s not really the money that bothers me, but the principle of what has happened makes my blood boil.

Please can someone advise me whether:
There is a case to be had?
How should I go about raising a case? (NB I have not made any contact with company yet).

Thanks in advance for any help