is there a legal requirement for a disclaimer to be added to email
under UK or EU law.

I guess you might wish to be more specific. Obviously, if there was a
law requiring a disclaimer, there are two problems. 1) Everyone using
email violates the law daily. 2) Proving violation violates privacy
laws.

However, many businesses attempt to reduce liability by attaching
disclaimers as footers to messages: "This message may contain
confidential information... only intended for recipient ... destroy if
you're not the recipient." I don't think they're required to do so by
law, but I guess it makes the lawyers happy.

This is a free comment.

I am not aware of any legal requirement to add a disclaimer.  Indeed
the Commercial Manager for the company I work for refused to add one
to his emails that said the email was the person's own views and not
that of the company's.  I would agree the disclaimer sounds daft.  If
I were in dispute with a company who issued an email under a
disclaimer that I wanted to make use of in a legal case against the
company I would say the email counted in court against the company
rather than the individual employee.  The judge would try to be
reasonable.  So, if the comment was of a personal nature and the issue
did not concern the business of the company, I would argue the
individual was the subject of my complaint rather than the company.

Bottom line for me is disclaimer counts for nothing.  The content and
context counts for everything.