It appears that in the UK hook sizing uses a numbering system (which is different than the physical metric size of the hook), which runs from 14 to 000--smallest to largest.
In the US, "aluminum" hooks are lettered, beginning with "A" as the smallest hook and going on through the alphabet, generally with a "P" being the largest (although, I do believe that I have seen a pattern requesting a size "S"!).
I have attached URLs which have charts to use for converting into US hook sizes.
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