The long-awaited London Metropolitan Archives records have begun to appear on Ancestry.com. London, England: Marriages and Banns 1754-1921 and London, England: Baptisms, Marriages and Burials 1538-1812 were loaded on 14th September. Marriages include the images of the banns record and marriage record if available. The easiest way to access these records is to click on List Recent Databases on the homepage and then click on the titles given above. The information comes from more than 10,000 Church of England parish registers from the Greater London area. The baptism and burial registers are not currently indexed. You can’t search these by name. You must scroll down and click on the most likely borough and browse rather than using the search boxes. You will do better if you have some idea of the dates and place you wish to search.
Ancestry.com (Library edition) is a subscription database that is available free to researchers within the State Library building. Some public libraries also subscribe to Ancestry.com so you may wish to enquire as to its availability at your local library. Happy hunting!