Happily, fares to London from the U.S. in October are some of the lowest, comparatively speaking. These days, however, you're not likely to find too much below about about $400 (depending on your outbound airport). Airlines typically start to roll out transatlantic shoulder-season deals in early September for travel completed by around the end of October. Virgin Atlantic, for example, is already offering some deals, though I'd be inclined to wait till the end-of-summer fire sales. You can view Virgina info here: http://www.virgin-atlantic.com/en/us/specialoffers/usspecialoffers.jsp

Here's a link to an article on Away.com with a few more tips when to book airfare: http://bit.ly/jkNsiP