If you are made redundant when furloughed, you will still be entitled to receive notice and pay. The amount of notice pay will depend on the amount of notice you are entitled to receive on termination of employment.
If you are entitled to the statutory minimum notice, notice pay is based on full salary (100%), not furlough salary (80% if that is what is agreed). If you do not have a written contract, you will be entitled to statutory notice.
If the contract gives at least one week more than statutory notice, notice pay depends on the furlough agreement. If you agreed to reduce pay to 80%, notice pay will be paid at 80% if notice runs whilst you remain furloughed.