The Electoral Commission is an independent body set up by the UK Parliament whose aim is "integrity and public confidence in the democratic process."
The website which contains information about candidate nominations, results of past and information about upcoming elections, a Referendums link which contains all the results of UK and national referendums; mayoral referendums; recent international referendums, from about 2005/2007. In addition this site maintains a series of searchable registers such as Register of political parties, Register of donations to political parties, Register of donations by donors etc from 2001 (downloadable to MS Excel format).
There's also a link to publications and research containing election reports, fact sheets and case studies, and policy and research resources.
Data available freely on the internet