There are a number of differences between bank holidays in the different parts of the United Kingdom - England, Wales, Scotland and Northern Ireland. However, KDE only supports a 'United Kingdom' setting meaning that a bank holiday can be announced in a country when it is not or missed because it isn't celebrated in England - which I presume is the basis for the KDE list.

I'd be happy to explore ways of dealing with this.

I am not sure who is working on Holidays in KDE currently, however the kde-core-devel mailing list may be able to put you in contact with the right person.

You may also wish to ask on #kde-devel on Freenode.