OK, so in the past I have been in a management team where we have offered a settlement (or compromise agreement) as opposed to redundancy to someone. I think from memory, this is the difference: -
Redundancy is due to the employer's need to reduce the workforce and the role will be made redundant, while a settlement (or compromise) agreement is a mutual decision to end the employment relationship, this means the role is not necessarily redundant but they want you gone without you being able to take legal action against them for it. This is not always a negative thing.
Redundancy follows specific employment laws and requires a fair consultation process, whereas a settlement agreement is negotiated between the parties concerned and is legally binding. This means that if you go for the settlement and it is done properly, you cannot takes legal action against them. As opposed to making you redundant against your will, when the job isn't actually redundant, you can then take legal action.
Both can include financial compensation, but the terms and amounts can vary significantly, because (in my experience) they will normally offer you more money to go in a settlement agreement. However if you are seriously considering this, you must ask that they cover your legal expenses for the settlement agreement. By this I mean they agree to pay a solicitor of your choice to represent you and your best interests.
They can force fair redundancy on you, they cannot force a settlement, it has to be mutually agreed and part of that agreement is that you do not have any further legal recourse.
It sounds to me that they know they do not have legal grounds to sack or make you redundant but want to help you leave - if you are in a Union, speak to your Union rep.