Due to the congestion and complexity of the route structure in the London TMA, tactical positioning of traffic with the use of headings is absolutely essential for an expeditious traffic flow. If all aircraft were routinely left on their own navigation the TMA would grind to a halt.
You may not require a heading to be separated from conflicting traffic in the immediate vicinity, but tactical positioning early on can prevent numerous conflictions from happening 20, 30 or even 50 miles ahead.
If you were overflying the TMA at FL100, you can be assured that if left on your own navigation, the result would be a significant reduction in the expedition of pretty much all the other traffic.