Yes, it IS necessary to refer to the Frisian and Low German
diminutives because that were the original forms brought to the Medieval England by Flemish wool weavers.
And I never said that J's are not pronounced like J's in the UK. I said that in _all the other languages_ I mentioned they are pronounced like Y!
had nothing to do with the French name Jacques
, but later and still today Jack
is often referred to be an English version of Jacques
because of the similar sound. You will find that in many books even if most of the authors correctly mention that the connection was made up much later.