In my experience, running a site on a
.co.uk site is better for ranking in the UK rather than a subdirectory of the
For one, domain authority and PageRank aren't everything anymore. Google has never published number for domain authority and they recently stopped publishing publicly available PageRank scores. You must be relying on third parties to provide you with these metrics. Those third parties are not Google. What they tell you about your authority isn't necessarily exactly what Google pays attention to.
When you have a
.co.uk site, Google automatically geo targets it to the UK (in fact, you can't globally target such a site.) When the UK site is in a subdirectory, the geo targeting is not automatic. You have to register the subdirectory separately in Google Webmaster Tools and set the geo targeting just for that subdirectory. It sounds like you have done this, but it may not be as effective as the natural
Having hosting in the targeted country is also important. Google appears to use where the site is geographically hosted as a ranking and localization factor for international searchers. It is not possible to move hosting of a subdirectory into the correct country. Because of this, I always recommend a separate site or at least a subdomain.
Users also seem to like sites that appear to be more "native". Users in the UK appear to be more receptive to
.co.uk sites than uk subdirectories or subdomains. This makes it easier to build links. It also raises the click through rate from the search results. Those factors can lead to higher rankings.
If I were you, I would change it back to the way it used to be.