Early decimal coins were mostly struck from Copper/ Nickel / Silver alloys and are nowadays worth considerably more as scrap metal than as currency, or as collectors items.
(More recent small-change coins are electro-plated steel, and have no scrap value.)
People with access to large quantities of small change will sometimes separate the early coinage from the later stuff with a magnet and weigh it in.
This should give you some idea of how good an investment definitive-pattern coins actually are.
And before anybody starts on about "Defacing the royal currency"...yes, you are allowed to destroy, deface, recycle, or do anything you want with a coin, provided you own it, or have the owners permission.