I read on airliners.net that Emirates bought their latest LHR slots for 4 mio. $ from United - so their increase in Heathrow is not a miracle but a question of money. OTOH this indicates that they prefer to pay 4 mio. $ for a LHR slot instead of taking free slots in Stansted. But they should give STN a try - with the proximity to the City there should be some potential.

As a coincidence, Virgin just rented a LHR slot from United for some years to start their new service to Dubai this year.