There is no mandatory list of contents for first aid boxes. Deciding what to include should be based on the employer's assessment of first aid needs. The Health and Safety Executive (HSE) leaflet First aid at work: Your questions answered recommends a list of contents for where there is no special risk in the workplace.
Any items in the first aid box that have passed their expiry date should be disposed of safely.
In general, tablets and medications should not be kept in the first aid box.
If mains tap water is not readily available for eye irrigation, at least one litre of sterile water or sterile normal saline (0.9%) in sealed, disposable containers should be provided. When the seal has been broken, the container should not be reused. The contents should not be used after their expiry date.