There is a British Standard BS 8599 for first aid kits, it is not  a regulatory requirement under the Health and Safety (First-Aid)  Regulations 1981 to purchase kits that comply with this standard.   Instead the contents of a first aid box is dependent on an employers  first aid needs assessment.
This means for employers following a needs assessment the options are:
1.    Within your workplace you have access to a first aid kit  whose contents complies with BS 8599 and matches your needs assessment;
or
2.    Within your workplace you have access to a first aid kit whose  contents matches your needs assessment but does not comply with the  requirements of BS 8599.