Yeah, I was the same until a machine failed and a colleague said that the coin might be fake.

There is something rather delicious in the fact that we're so used to machines and computers failing that the idea the coin could be wrong never entered our heads!

Prior to about 2010, 5p and 10p coins really were thinner. They were changed from copper/nickel alloy (Cupronickel) to nickel-plated steel to reduce costs. But in order to maintain the same weight, the coins had to be made thicker.