Thank you. I am afraid there are no legal grounds to challenge this in the circumstances. The statutory redundancy cap is set by law - there is absolutely nothing the employer can do in relation to this. It is not their choice that you are capped at 20 years for redundancy payments and they are only following the law.
As to the enhanced packages offered in the past, as you have identified these were offered for VRs, and even if they were not, just because they have been offered in the past does not mean they are now a guarantee for all future redundancies. Unless you have an enhanced redundancy package entitlement under contract you cannot expect one to be offered by the employer and this would be done voluntarily if they see fit.
Sadly there is nothing illegal in two employees, one who has worked for 20 years and one who has worked for 40 years, to get the exact same payout in a redundancy situation. That is the current law and the employer is not doing anything wrong by following it, even if it appears unfair.
There is of course nothing stopping you from asking for more and negotiating with them but if they really wanted to they can stick to the statutory redundancy entitlement and that cannot be challenged because it is the law.
I'm sorry if this is not necessarily the answer you were hoping for, however I do have a duty to be honest and explain the actual legal position. This does mean delivering bad news from time to time. I hope you understand and would be happy to provide any further clarification if needed. If you are still satisfied with the level of service I have provided regardless of the contents of the answer, I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 stars above. Thank you