You may have been subject to a departure interval suddenly imposed by En-Route. Normally the flow off Heathrow say is 1 a/c every 2 minutes on the same route, speed dependant. There are a few hours in the morning and evening where this increases to 1 in 3, and sometimes during heavy traffic En-Route will impose 1 in 5 without warning due to high demand. Then I would expect you to have to move aside and let some through.