@mpb okay i see now what the problem is. checked the UK localization file and the report transformations still fetch data using the custom fields for tax codes. this was recently changed and Reporting Categories were introduced.
you can read the other localization topics here where there has been discussion about the same. this will help you understand how to use the reporting categories to create report transformations. also, it would be helpful to other UK users if you can update the localization file for UK as per the recent changes.
there is a dedicated topic for UK localization. all discussion regarding UK localization should be made in the topic.