Have a look at the thread for outland London and it's spreadsheet (the link is in the signature of several members on the thread). The spreadsheet has the timelines for forum members who sponsored through London going back to 2010 and shows averages at the bottom
Thread: http://www.canadavisa.com/canada-immigration-discussion-board/thread-for-outland-london-uk-applications-t49436.10620.html
Spreadsheet: https://docs.google.com/spreadsheet/ccc?key=0AvI5l_hMO7UKdHc2LUJuZ2Itcld2Vk1WR25GeVZxYVE#gid=0