First, how is the European standard of "organic" defined exactly? Both the UK's and US' seem to cover the basics (no growth hormones, limited herbicides and pesticides use, must feed the animal organic foods, etc.), but fertility and biological standards in the soil might vary. It is possible that that UK might have naturally less-polluted soil than the US (fewer heavy metals?), contributing to the difference. Also, organic doesn't mean hormones or pesticides-free; rather, it just defines which pesticides can be used and are not as detrimental to the environment. There might be different standards here too.
UK's standard: https://www.gov.uk/converting-to-organic-farming
US' standard: http://www.ams.usda.gov/AMSv1.0/NOPOrganicStandards
Secondly, most organic studies in the states dive into which fruits/vegetables are subject to the most pesticide use. Apples, strawberries, and grapes seem to have the most in the US, but avocados, corn, and pineapple have the least. Studies that make blanket statements like "organic foods are better for you" without acknowledging the differences in the cultivation and biology of different species are lacking in my book.
Extensive List: http://www.ewg.org/foodnews/list.php
I'd buy organic, mostly for the more humane way farmers treat livestock and lower environmental pollution, but I'm still not convinced that organic is nutritionally superior or has levels of pesticides that prove harmful to humans.