Can employers prevent staff from holidaying abroad to prevent quarantine chaos?

In short, the answer is no. An employer does not have the right to prohibit an employee from going abroad in their own time, nor can they specify which countries an employee may travel to. Of course, an employer may make recommendations, but these will not be binding. Further, an employer does not have the right to force an employee to disclose where they are going during legitimate leave periods. 

In line with internal policies in each company, employees will (usually) request holidays in advance and employers can retain the right to refuse or cancel holiday requests. Typically, there must be a legitimate reason for refusing/cancelling and the employer must give an appropriate notice period (typically the same amount of time the employee has requested off. For example, an employer must cancel a holiday period of 5 days no later than 5 days before the holiday was due to start). Whilst not tested in courts yet, it may be the case that an employer could successfully argue that an employee needing to avoid quarantining may amount to a legitimate business purpose. However, if holiday is unreasonably denied then it could amount to constructive dismissal or claims of discrimination. 

Setting the law to one side, refusing or cancelling a holiday is likely to be a highly unpopular decision and can have long term repercussions in employer-employee relationships. If the employee can work effectively from home during a potential quarantine period, then it seems sensible to permit the holiday. The bottom line is that employees are entitled to holiday, and it should not be unreasonably withheld. If they legitimately cannot work remotely, then a discussion needs to happen regarding how they will be paid should a quarantine period become necessary. Assuming that an employee is not unwell during a quarantine (with Covid or otherwise) then they will not be entitled to statutory sick pay. Therefore, if an employer is not willing to compensate them for their quarantine, then to receive payment whilst not working, the employee will have to use up more of their holiday entitlement. Failing this, an employee will have to take unpaid leave. There is a further possibility of an employer being able to take disciplinary action should the absence due to the quarantine be an unauthorised one. It is advised that this is explained clearly to the employee before they take their holiday so that they can weigh up the risks and benefits themselves and make the most informed decision possible.