jecuk
Your best answer would be a visit to Swanwick to see how the overall system works. Basically there is a large number of sectors split both vertically and horizontally like an irregular pile of Lego bricks....and your route will take a path through the most efficient line of those bricks. What do I mean by efficient? The system is optimised for a Huge and Vast throughput of traffic almost beyond comprehension....rather than a 5-star service for just a few movements. In order to achieve maximum airspace usage there needs to be a regulated system of ATC handling....if you study charts and routes around the "south east" you will start to get the picture.
There is more flexibility outside peak hours....and especially at night. Don't take this the wrong way, I'm not being cheeky, but controllers are familiar with the operating characteristics and profile preferences of a wide spectrum of aircraft types....it's our job....and you will not be given premature descent without good reason.