I used this link to get wt+ bos-lhr for $500 inc. taxes in Oct... looks like many other cities are included...it's the friends&family discount.

* Round-trip, 7-day advance purchase, non-refundable, select economy class fares and select "T" class fares for travel Sept. 05-Dec. 15, 2005 and Dec. 24-Mar. 31, 2006 from New York JFK and Newark to London on British Airways only. Higher fares apply for other travel dates, from other departure gateways and to other European destinations. Fares subject to gov't. approval & include gov't. fees/taxes ranging from $110 to $144 and a $2.50 September 11th Security Fee. Min. stay Sat. night required. Max stay 11 months. New bookings only, made Aug. 23-Sept. 05, 2005, subject to availability. Other significant restrictions apply. For full terms and conditions, click here.