Please listen to the advice of @prh47bridge OP
SA’s have their place in a workplace and whilst not nice to receive, they shorten a horrible process which can sometimes be long, drawn out and with the same outcome (exit). If handled correctly by the employer, they’ll have put aside a figure additional to redundancy, to pay you off. That won’t be the figure you’ve been given. Somehow you need to reach that figure through negotiations. It’s a dance between the two parties.
Get a solicitor (a good one, search recommendations from people in your area). They’ll negotiate for you.
I’ll be honest, the employer may play hard ball and not budge on their original offer. I have been that person because I’ve had simple cases where I know we could do the normal exit via redundancy relatively easily, so it would be no issue to refuse the amount put forward (by employees solicitor) and state the process will commence day after the deadline to agree it or not. This results, as others have said, in statutory redundancy payout only which you want to avoid.
BUT, I’ve also been negotiating when we want the person removed from our business; and it’ll be a drawn out process, damaging for morale, high risk of unfair or constructive dismissal etc, if they remain beyond the SA process (normally 10days) so have negotiated but made sure the person has been given a fair settlement so that they take it without a fuss.
Aside from money, it’s worth pushing for other things such as: glowing reference (you can write it yourself), employee outplacement service (c£500 with a decent HR consultant to help you via coaching, cv review, job search etc) missing bonus that you may have achieved, holiday accrual correctly calculated. Decent solicitors will pick up on these aspects.
Sorry you’re going through this. I can recommend a solicitor if you need one. She’s ours for our company but she has provided advice to people I know (as employees) who have had SA’s issues.