As far as I can make out, some kind of geo-filtering seems to be going on. The UK search is quite similar to the generic .com one, but lots of sites are missing. Now the thing is, there doesn't seem to be any pattern in it. Most .co.uk sites remain in the results, yes, but so are many .com sites (some of them clearly US based).
Has anyone got any clues?
View the location algo boost as a "plus 10 points" factor. If it is also reasonably optimised it may wind up with a standard optimisation factor of 20 - plus 10 for being local, = 30 points. It will beat other sites which are less than 30 points. However, if a site has 35 points for optimisation and other off page factors - then it will beat the local site.
Note all these figures are for illustrative purposes only :)
But, hopefully, it will give you the idea.