I appreciate from the answers that you provide on here that you know quite a bit about this, so I apologise if this goes over old ground. The best place to start is probably here - the BERR website.
http://www.berr.gov.uk/whatwedo/employment/emp loyment-legislation/employment-guidance/page13 852.html
The process started (I assume) on 19th May, which was the day you wrote the question. Isuggest you work through these points carefully if you haven't already done so - it isn't the actual statute law, but it's an excellent interpretation of the actual law.
I also assume that collective bargaining rights don't apply, but then you say you are being nominated as the 'appropriate representative of the employees'.
Firstly, then, when was this 'loyalty bonus scheme' mentioned? - presumably some time before the start of the current process? Were any figures mentioned? - the trouble being that the company could turn around and say ' the bonus is �10 to each person, pal'.
Secondly, there are minimum bits of information that must be supplied (to the reps) at the start of the 90 day process - see half way down the BERR page under 'What Information must be disclosed'. Have that little lot been supplied to you by the employer?
I'm afraid that I don't think there are any precedents regarding payments more than the minimum statutory amounts - unless it is mentioned anywhere that could be construed as part of T&Cs (many public sector departments have this - not so in private sector). i.e. just because the company paid X amount last time a redundancy situation applied doesn't mean it has to do the same again.
Thirdly I'm afraid I believe that the 20 year rule is what the statutory legislation caps the payments at. Of course there is no obligation for the employer to limit the payments at 20