This kind of policy seems to have become more common in the last few years, requiring employees to notify their manager of any relationships that start at work. The idea behind it in most cases is to make management aware of possible conflicts of interest - If a staff member and their line manager become an item, for example, there is the potential for favouritism in promotion or performance reviews.
A blanket 'kiss and tell' policy isn't good HR practice though. It's a heavy-handed measure, which doesn't treat staff as responsible adults. There are also questions about how enforceable it would actually be. At what stage should you declare it - on the first date, or after your engagement party? If you are dismissed for not revealing a relationship, you may have a case for unfair dismissal at an employment tribunal (though be sure to take proper advice before any legal action).
Even if you don't reveal a relationship you won't lose your rights to protection from harassment if you then suffer it as a result of that relationship breaking up.
