And don't forget the circuit height is 1100ft, so you'd only be a maximum of 400ft above it.
Technically, that would put you inside Stansted's airspace (1500 feet and above)! In reality it's 300 feet seperation.
However, dont forget jet circuit traffic at NW flies at 1000 feet AAL (1300 QNH) so your seperation form them is only 100 feet!