See UK IAIP ENR 1-1-4-2 Para 1.7
1.7 Single-Engine Fixed Wing Aircraft Over Central London
1.7.1 With the exception of the Northolt RMA (including for Runway 07 the portion of the London Control Zone north of the offset centre-line) (see AD 2-EGWU AD 2.17) and the Local Flying Areas at Denham and Brooklands (see AD 2-EGLL AD 2.22), NSF or ENSF permissions will not be granted to single-engine fixed wing aircraft requesting to operate within those parts of the London and London City Control Zones between a North-South line extending through the LON VOR and a North-South line extending through the LCY NDB. In accordance with a directive from the CAA Directorate of Airspace Policy, applications which fall within the above criteria will be refused upon application to the NSF Co-ordinator due to the inability of such aircraft, in the view of the CAA, to be able to comply with the ANO Rules of the Air Regulations 2007 - Low Flying (Rule 5d).