The Government Equalities Office has recently issued new guidance on dress codes and sex discrimination. Whilst it has long be agreed that dress codes for men and women do not need to be identical, any standards should be equivalent.
Further, employers should avoid gender specific prescriptive requirements, for example the requirement to wear high heels. In particular, any requirement to wear make-up, skirts, have manicured nails, certain hairstyles or specific types of hosiery is likely to be unlawful.
A full copy of the guidance may be found here.