Plus you don't need to do the aircraft work for a 5.5 deg GS at London - just the operational stuff for London.
You can do the "raw" cert for a 5.5 deg GS wherever you like - as long as there's a suitable GS. Since you also need a 7.5 deg GS somewhere (to demo the 7.5 deg cert capability noted above) it may be a LOT easier to find somewhere quiet that will reset their beam to an angle of your choice. That way you dont get tied up with normal London City traffic etc.
Chances are Airbus did at least as much as Embraer - but at some other location.