Isn't the caption 'Intense Microlight Activity' on the 1:500,000 chart 2 miles off the flight path sufficient to suggest that this wasn't a sensible place for an A319 to be at 2000', even in perfect VMC?
Immediately prior to the airprox, the A319 passed very close to Rochester's ATZ, below the 3500' base of the London TMA in an area often busy with GA aircraft Indeed, the airprox report shows that the Airbus was advised of two GA aircraft passing close to it.
Sadly, the passengers of the Airbus are unlikely to be aware of the increased risks they are taking using Southend so that their operator can pay reduced costs compared with using the nearby Stansted which has plenty of spare capacity - and controlled airspace.