I work in the UK niche for most of my sites, key points to consider- Is the hosting located in the UK (check the IP) Have you told GWT to target the UK Also the reason you might be ranking higher in the US is there could be lower competition or you might be targeting a market that has not yet emerged stateside.
To get good index for UK, you need to get some authority link from UK. Another thing is your server location. If your target is UK, better to get one there.
GWT is set as UK on geo graph the competing sites are approx the same for both google.com and google.co.uk its so bizzare why this is.... that i get a better ranking on google.com as oppose to google.co.uk the hosting must be a crucial component