http://www.nats-uk.ead-it.com/aip/cu...8_Y_092_en.pdf
2.1 A pilot flying around the peripheries of London City, London Gatwick, London Luton and London Stansted Controlled Airspace,
as depicted on the attached maps, that does NOT require any air traffic service may elect to squawk a discrete SSR code as indicated
in the table below with Mode C selected if available, whilst monitoring the relevant frequency.
London Luton [and] London Stansted : 0013
Chris N.