Your employer will need to hold a disciplinary hearing before dismissing you.
If you cannot attend the hearing because you are on sick leave (or call in sick on the day), the employer might decide to reschedule it for another day. Or if you are on long-term sick leave, the employer might suggest alternatives: for example, asking you if you can attend the meeting by phone or send a union representative instead. Or the employer might go ahead with the hearing anyway, leaving you to put forward any objections at an appeal hearing.
In practice, this means that sick leave may delay any dismissal but not prevent it altogether.
As far as being made redundant is concerned, you need to be selected for redundancy fairly and a fair redundancy process must be followed. You could claim unfair dismissal if your employer doesn’t consult with you about the redundancy, or if you are selected for redundancy because of your illness.