'Congested area' in relation to a city, town or settlement, means any area which is substantially used for residential, industrial, commercial or recreational purposes;
Strictly interpreted this rules out pretty much all of the UK, or at least the southern part
I think the issue here is the definition of 'substantially'. I know the area quite well from the ground (never flown the corridor), but it's a completely different ballgame - quite literally! - on the weekends. The marshes are totally deserted during the week and crowded on the w/e.
I think it's one of the instances where the rulemaking body leaves itself wiggle room on purpose. Interestingly, it's exactly the same in the US - the FAA stubbornly refuses to clarify the meaning of 'congested area'.