This very much depends on why you were being asked to enter into a settlement agreement in the first place and whether your employment has already terminated.
If you chose not to enter into a settlement agreement but you have been unfairly dismissed or have other employment related claims, you can bring a claim before the Employment Tribunal (or County Court if it relates to contractual payments) if this claim has merits and you do so within the necessary timescales. Alternatively, you will need to commence ACAS EC before you start Tribunal proceedings if you have not already done so, so an ACAS officer is appointed and can liaise with your employer.
You may be offered a settlement under a COT3 instead which is another form of legally binding settlement agreed via ACAS and you do not need to have legal advice in this scenario. ACAS provide an independent person to liaise with your employer for you but they cannot provide legal advice just guidance.
If the settlement agreement was in an attempt to come to an amicable end to the employment relationship but you have not been dismissed then your employment will continue until you resign or action is taken against you such as dismissal. If this happens and the settlement agreement process has been handled correctly then you will not be able to refer to the fact that you had a draft settlement agreement or the terms in any future process such as employment tribunal claims if it is an unfair dismissal claim but your solicitor will be able to provide advice on this at the relevant time.