You will certainly have rights in this case. If your holiday was pre booked and formally approved by your employer then they can't really go back on that and cancel it. The fact the you have been off sick just before that should not change anything. This time should not be treated as time off by your employer as it was not planned or unauthorised and it is genuine time off due tao sickness. As such, it should not be taken into account when considering how much time you have had off in terms of holidays. Sickness and holidays are separate issues. Therefore you should not be penalised in terms of holidays if you have been off sick and can provide proof that it was genuine sickness. You are entitled to take the pre booked holidays you have been granted as it is now too late for the employer to cancel these. 

The problem may be if the employer treats this as unauthorised time off work and then take disciplinary action. It's a possibility and can't be prevented if the employer is intent on doing it but you can challenge it and even raise a formal grievance if necessary. 

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