In a cafe I would never expect to see a
metal detector; apart from the fact your dishes are probably made in metal tins anyway, it's just not practicable. The off-site baking facility might be a different matter and as Charles says, whether it's seen as acceptable by the authorities may, in part be associated with whether competitors of a similar size have them.
But with
HACCP it's about risk and if you want to assess the risk yourself, I would first look at maintenance records and audit machinery. Are nuts and bolts missing? How confident are you of your maintenance controls that if bits did fall off the right procedures would be followed? (If not, the solution may be to beef these up.) Secondly, have you ever had any metal complaints? Thirdly, why not trial it? Contact a
metal detector manufacturer and test it out, they may hire one to your or lend you one for free if you look as though you might buy one (hint... look enthusiastic even if you're not sure

). See if you do have an issue.
That all said, if you are a manufacturer of any real size, I suspect most EHOs in the UK would expect a
metal detector nowadays simply because they are cheap and do the job. Your question is whether you think it's worth the effort to convince them otherwise. The downside to not having one is if you did end up with an enormous bolt in a loaf of bread which reached a consumer, in the UK anyway there's probably very little you could do to convince a court that you'd done everything you could to prevent (ie due diligence) without having a
metal detector but as Charles says, that would be something which would be very dependent on size of your business.
What are we talking for a
metal detector nowadays? Around £2k?