It is also a question of wind direction. For takeoff, an aircraft can handle a much higher windspeed on the nose - than from the side angle. You may notice that many large airports have runways at different angles of the compass to allow for changing winds.
As did Heathrow when it was built. But the limits of their expansion meant just two parrallel runways are left. So, if the runway was at a different angle, they might be able to use it in particular winds.
As we know, Heathrow has been restricted to two runways (and always will be) but even when they planned a third and fourth - they were still parrallel to use the prevailing winds as it used up less land. Check the runway layouts of Schipol and Frankfurt.
That said, the winds have been exceptional this month.