Is there a data dictionary?

Metadata and further information about each field can be found in Showcase Catalogues.  If you Search in Showcase for a field number or a keyword, and select the field page, some of the information will be visible. Further details can be found in the various tabs (Data/Instances/Notes/Related fields/Resources).

See the "Category Notes" for each category that the field is in. Categories are listed on each field page, under the Data-Field Description.

Some of the resources are also available via the UK Biobank RAP, often as helper files.

To work with metadata programmatically, there are some notebooks available in:  https://github.com/UK-Biobank/UKB-RAP-Notebooks-Access