During health and safety leave, you are deemed to still work for your employer. Your employment rights, such as annual leave, increments, seniority - whether they are given by law, under the terms of your employment contract or otherwise - continue to build up as if you were not absent from work. In other words, these absences count as reckonable service, so your employer cannot count them towards any other leave, such as annual leave or sick leave, to which you are entitled.
While absent on health and safety leave, you cannot get time off in place of any public holidays that occur during your health and safety leave. However, you may keep your entitlement to annual holidays.