You may be presented with a settlement agreement by your employer completely out of the blue. This is more likely to happen where your performance is brought into question, and your employer wants to give you the option to leave under agreed terms rather than go through a performance process.
You do not have to accept the offer. You can reject it, or request a proper performance process is followed.
The ACAS Code recommends that you be given a period of 10 calendar days to consider an offer made by an employer, but it can be less if this is considered to be reasonable.
The Code also gives examples of “improper behaviour” associated with reaching a settlement including putting undue pressure on you to agree an offer. This includes, for example, an employer saying before any disciplinary proceedings have begun that you WILL be dismissed if you do not accept the offer, and also all forms of bullying and harassment and intimidation.
You will need to be careful and guarded if there is an unexpected approach by your employer with an offer for you to leave. It can be difficult to negotiate a figure upwards once you have already agreed to it (even though terms are not binding until you have taken legal advice on the Settlement Agreement). It is not wise to even provide the most basic signal that you are prepared to give up your employment.
Unless you consider the offer is too good to turn down, you should preferably just listen to what your employer has to say at the first meeting without committing yourself either way, and then take immediate legal advice.