There seems to be an agreed standard for everything! I’ve just been checking our first aid kits against the current British Standard 8599-1:2011, for first aid kits in the workplace, and interestingly there are some differences. The British Standard isn’t a legal requirement – I understand the provision of first aid kits is governed by The Health and Safety (First-Aid) Regulations 1981, which only requires “adequate and appropriate equipment”, but the British Standard is a pretty good starting point.
I suppose additional equipment may be required depending on the organisation, task or people involved. In particular, the provision of specialist burns dressings is required to meet the British Standard, but are often omitted in kits available commercially. There is also an associated British Standard for vehicle first aid kits.