Issues around dress and personal hygiene are always difficult conversations that many employers simply hate to have. Having a dress code in place spelling out what many would see as obvious can assist with ensuring that staff have a uniform approach to dress at work even if this is not a uniform in the traditional sense.
ACAS have issued guidance for employers about Dress Codes. It does not provide a policy but more some of the issues that employers should consider including some pitfalls when it comes to discrimination. It also talks about body piercing. You can read more here.