Because they are not pure copper

well, it would appear that your one penny and two pence coins are partially comprised of steel… a ferrousmagnetic material.

oh, and the current coins appear to have been introduced in 1971. You’ll hafta dig for yourself to find out if steel or other magnetic ingredient was a component prior. ;-)

your 5 cent 2 cent and 1 cent coins are copper covered steel coins
http://www.euro.gov.uk/eurocoins.asp

I was wondering the same thing the other day when I was at coinstar and the machine got jammed because I had accidentally put a bunch of European coins in there. A magnet caught them.