Took a quick look at Worldcon 2014 hotels, and the two nearest are >$400 a night. That's the AAA discount rate. Travelodge a couple of blocks away is just under $140 a night. I have cousins who live near Heathrow and I think one in London whose home is a B&B, but I'm not sure if it's a real B&B or if her kids/grandkids just jokingly call it that. But those are both too far to get the full convention experience. I'll add time to my trip to see them, of course.
Actually only one of the hotels was open to bookings for 14/8/14 - the others I used last week of July dates.