Do you not get the "Pages from the UK" filter below your location on the left of the results page?
In any case, since Google dropped the equivalent to that option that used to be below the search input (where it should be!), it has relied on its understanding of its own results coupled with the need for website owners to do the right things to ensure that their location is recognised and target market correctly worked out by Google.
That has, as you've found, sometimes been less than perfect, but it's the way in which search is going. It will get better, but fine tuning your own way of searching (to keep up with the changes) can help.
Some results that would be relevant won't always be well placed, while some irrelevant results may be prominent: that's just a reflection of the enormous complexity involved for all concerned and the fact that the picture is constantly evolving.
If you use Google Toolbar to do your searches (only available if you are using Internet Explorer or Firefox), one of the buttons on the toolbar (when installed in the UK) is UK - Find pages in a country.