There were many things that did in the idea of Boris Island from a poor design point of view, but one was surely that it's runways were still East/West, meaning that aircraft would still pass over the metropolitan area, especially arrivals when they were on Easterlies. And with multiple parallel runways you typically need longer runs in when you are doing parallel approaches.
Why were the runways not north/south ?
Paris, for example, gets it right with both Orly and CDG being north and south of the metropolitan area, with runways east/west so traffic does not pass over the city.