Was it possible to rotate furlough leave so employees were furloughed at different times?

Yes. Employees could be furloughed multiple times, which meant that they could be furloughed, brought back to work, then re-furloughed.

Could an employee be furloughed if they were on sick leave?

No, employees on sick leave or self-isolating were eligible for Statutory Sick Pay (and, if applicable, contractual sick pay). When they were on sick leave or were self-isolating, employees could not be treated as furloughed under the scheme but they could have been furloughed afterwards.