You ask about other decimals that are hard to find Eric and this 1970 halfpenny proof is although a variety ,which was mentioned the other week.
D to gap i to tooth.
The lowest mintage Great britain 50p (not struck in error) is the blue peter 2009 with less than 20,000 minted but they are at upto £250 /300 now.Most of the low mintage and collectable decimals are the territory coins of mainly Gibraltar and the isle of man.The rarest or hardest currency to find being the 1989 isle of man blimp £2 and the last one i saw sell was for £2000.