Companies can and do develop HR policies which specify rules for relationships at work. But there is a difference between requiring staff couples to behave in a professional manner whilst at work and banning relationships altogether.
This is bad HR practice and is only likely to make staff keep relationships secret. A policy this draconian may conflict with the Human Rights Act 1998, which ensures the right to a private life, even when at work.
An employer who doesn’t treat all relationships the same – for example, by taking a different attitude to heterosexual and same-sex relationships – will be guilty of unlawful discrimination. No service is needed for this kind of claim.
In some professions, a policy banning relationships is likely to be defensible when used to protect against relationships that are – or can be perceived as being – inappropriate and a betrayal of professional trust. Common examples include between college tutors and students, and between doctors and patients. You should seek individual legal advice from your union or other sources if you find yourself in this situation.
Rather than a complete ban, some employers use policies such as requiring any office relationship to be disclosed, and then insisting that one of the parties moves to a different team or department, or report to a different manager.