A. These are not particularly widespread in the UK and are more commonly used by American owned employers. This type of policy tends to ask employees to disclose relationships to their employer so that safeguards can be put in place.
Some businesses require managers to disclose personal relationships with anyone in their direct line of management. As a business, you would need to consider whether it is fair and reasonable to dismiss an employee who fails to disclose a personal relationship. One area of difficulty is defining a personal relationship and when the duty to disclose is actually required.
If the threshold is not clear to employees then individuals might not consider they need to tell their boss about a relationship at the early stages. If considering a prohibition on relationships at work, you would need a clear policy , to consider the sanctions you would impose for non disclosure of the relationship, whether dismissing is fair and reasonable given the usual unfair dismissal rules will apply and consider issues of discrimination if, for example, one employee of one gender is disciplined or moved out of a department and the other is not. There would also be data protection issues to consider around who needs to be aware of the relationship and what they will do with the information.