I guess a lot of older style OpenSearch plugins (.xml format) were not migrated to the new style (.xpi format).
You can find .xml OpenSearch plugins for various Google sites here:
Meanwhile, I created a new extension (still under review) which uses the google.co.uk URLs from the old OpenSearch plugin. It should eventually be available here:
Note that I didn’t put the “G” icon in there at this point. Hope to add that later.
Next, whenever you want to search with Google UK just type “uk” in the omnibox and the search query.
The keyword property of a bookmark acts as an alias for the complete url. And the parameter %s will grab any text you enter after the keyword. You can assign a keyword to any search field by right click > Add keyword to this search. And you’ll find a new bookmark with the search details.