In many respects Corbyn wasn't boring enough. The Brits don't tend to go for the American President type, the celebrity leader; we tend to go for non-descript people who have the 'perfidious Albion' character. This is evident in Churchill, for example; not exactly a looker, had a drinking problem, but knew exactly how to deal with the threat at hand. After that, we needed to vote him out because he made a terrible peace-time leader and wanted to go to war with the Soviets. Churchill was not really a good character and this was known at the time.
Corbyn is a fundamentalist, on the other hand, and led a fundamentalist party within the Labour Party. Brits generally don't like that kind of thing and realise that it doesn't work within real world politics. I don't often like doing things for practical over moral purposes, but most political leaders need to do the practical as well as the moral in order to lead and deal with various nations. Churchill did this by allying with the Soviets against Hitler, but once Hitler was over he wanted to derail the Soviet train as well. Given the absolute state of Europe at this time, that was a death wish - it was Churchill's moralism taking over his more practical ethic. The Soviets would have crushed us. Corbyn strikes as the same; he promised what some people called the 'magic money tree' and to me, a capitalist, this is how it seemed. He was political friends with all the wrong people (he called Hamas and Hezbollah 'our friends' and no matter how hard he tried he couldn't backtrack that one). Hamas has been declared a terrorist organisation by the UK and his comment just went down like a brick.
Corbyn came off as a raggedy wonderworker who seemed to get in with all the wrong people, advanced anti-Semitism in his party and was generally talking fairy tales. He didn't seem to realise what the current political trends are. He would have been a disaster as PM.