A post at the Search Engine Roundtable Forum has detailed how you can see which pages of any UK site are found in the supplemental results on Google.

Just do a search on www.google.co.uk for site:www.yourcompany.co.uk and then click to view UK results. All of the results are those pages of the site that are found within the supplemental results.

Useful to know but I am not sure if it is actually supposed to work in this way. However until it is changed it is a quick way to find out which pages of your site are in this index