[dfawcus]

For 10p, 5p and 2.5p quantities, that wouldn't be an issue as following decimalisation, those simply reused the Florin, Shilling and Sixpence coins.

Also yielding:

  0.5p = 1.2d
    1p = 2.4d
    2p = 4.8d
    3p = 7.2d
    4p = 9.6d

I wonder how they handled those values with their mapping layer? Did the add a layer for handling fifths of an old penny?

(The 2.5p coins, then eventually also the 0.5p "new pennies" went out of circulation.)