I can tell you about the ones from the tower casinos. They had them on the blackjack tables in the Reading (and other) casinos. You would purchase it from the dealer much like you would a value chip. They did not sell them from the cashier - they redirected you back to the table where they had the commemorative.

You could not play the chip (or at the very least they discouraged it) but they would pay you in the commemorative on request (and redeem them at the cage for £5). You could then take the commemorative to the cage where they would punch it and provide the rest of the parts for a keychain.

They removed them from the tables in 2002 at the Reading Sporting Club when it was still a Tower casino.