One London hotel has been in the news because of the requirements it is reported to have issued to female staff.
Among these, apparently: shave your legs, use deodorant, and have regular manicures. Don’t have oily skin or wear overly garish or bright make-up.
Dress codes and appearance is a notoriously tricky legal area for employers to navigate. You are entitled to promote certain standards at work, and it can be really important to do that. The danger comes where there is no legitimate reason for requiring people to look a particular way, where the requests are unreasonable, or where they are more onerous to one protected group (men, women, older people, younger people, disabled people etc.) than another. A particular hotspot is requiring people to look a certain way when that is unrelated to their ability to do their job. Sex discrimination is an obvious possibility, but religious and other forms of discrimination could also bite.
This doesn’t mean that you can’t specify certain requirements, just that those requirements must be carefully judged and properly implemented. They are best formalised in a well-written policy that sets out, in a non-discriminatory way, not just the employer’s expectations but also the reasons for having the dress code in place. That could include health and safety, and the importance of a professional image.