Can an employer pay a furloughed employee 80% of their wages if this is below the national minimum wage rate?

Yes. HM Revenue and Customs guidance on calculating employees' wage costs under the Coronavirus Job Retention Scheme confirms that, as a minimum, furloughed employees must be paid the lower of 80% of their monthly earnings or £2,500, even if this brings their average hourly payment (based on their usual working hours) below the national minimum wage rate. This is because employees are entitled to the national minimum wage only for the hours they have worked, and they will not work any hours while furloughed.