High Quality Content by WIKIPEDIA articles! The Henley by-election, 2008 to elect a member of the British House of Commons for the constituency of Henley in Oxfordshire was held on 26 June 2008. It followed the election of Boris Johnson as Mayor of London in May 2008. In order to resign from being an MP, Johnson accepted the post of Steward and Bailiff of the Manor of Northstead on 4 June 2008.