Ha ha! Happy to oblige. The bellend or bell-end is the end of the penis, the helmet as it's a.k. But it's never used in an anatomical sense: it's mildish (though certainly offensive) UK slang for a stupid person, more or less equivalent to wanker.
"Bottom line is, some bellend in an office has recieved your letter and has responded in a heartless way due to either A) Being a twat or B) Knowing that there is no option and they're 'just doing their job'." - forum post, kumb.com, cited 6 March 2010.