Pretty much all of the airspace in the UK is busy, not just with UK traffic but also with high altitude traffic from Europe to North America. Airspace is three dimensional so it is pretty complex to navigate around as well as up and down.
And not just for airliners, trust me flying as a private pilot anywhere in and around London is either prohibited or extremely demanding. Why? Because we have to keep (and are kept by ATC) out of the way of all of the rising and descending traffic into all the London airports.

And to further respond to your question, there are peaks and flows at different times of day that affect routings as well of course the need to avoid dodgy weather.