The best answer that I can provide is that the traditional counties of England (and other parts of UK) don't line up exactly to the standardized hierarchy of places in Tableau's Mapping Service.
The good news is that there are many Tableau partners and customers creating custom polygon files to address this and make them freely available. You'd just need to blend the polygon TDEs to your data.
Here is a link from one of our UK partners that should explain it satisfactorily (and provide the TDE you need):
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Tableau Online Deployment Advisor