Or so was told recently be someone who deals in metals. He also said the same applies to some US coins.
Anyway, UK copper coins are being replaced with a copper coloured steel alloy. Seems about half of UK “coppers” are real copper and half are now steel and you can tell the difference with a magnet.
This magnet picked up steel coppers from a distance of at least a centimetre.
As for real copper coins, the magnet didn’t want to know anything about the coin, even from a distance of 3mm or so. And as far as the copper coin was concerned, the sentiment was entirely mutual.