Where an employee has taken voluntary redundancy, is the employer prevented from re-employing him or her in the future?

No, there is no restriction on an employer re-employing an employee who has taken voluntary redundancy. 
If the redundant employee is re-employed by the employer, he or she does not need to return any statutory redundancy payment, regardless of whether the offer of re-employment occurred immediately after his or her last day, or at some later date. However, receipt of a redundancy payment breaks continuity of service for the purposes of entitlement to further statutory redundancy pay in the future.

If the employee is offered re-employment prior to termination of employment taking effect, and the new job starts within four weeks of his or her last day of employment, the employee is not