How the thesauri compare depends on the point of view being taken. In terms of searching I would prefer BHI far above the UKDA. As a user the method of selecting and combining the terms was one of the best that I have used. The lack of ability to search multi-word entries in UKDA is one of its main difficulties. That and the lack of a help function at all. If the user was not sure of how the thesaurus worked, the BHI has a distinct advantage. Put these together with the flexibility of being able to choose what display you view first and the BHI has a higher value to a regular user.
Thinking like a professional indexer the value of the thesauri is not as divided. The idea that UKDA has more relationships would be helpful when trying to navigate through the thesaurus to uncover the best fit for indexing. The easier to read term entries and the direct link back to the thesaurus from the database entries would be more useful when indexing. Overall, both databases have positive and negative points. It would be interesting to try to amalgamate their best qualities together. That would create a very useable thesaurus.