I have operated a few parachute dropping flights over London with Islanders so might be able to help.
For any activity such as aerial photography over London you would need to speak to London Air Traffic Control Centre (LATCC) at West Drayton. You do not need to file a flight plan as the flight would be Special VFR. However, you will need to file non-standard flight (NSF) notification in writing. This needs to done well in advance although I am not sure how many days/weeks notice you would have to give. On the day you talk to Thames Radar or Heathrow Special. They will try to accommodate your request, but nothing is guaranteed as it depends on traffic and workload at the time. Also remember you will need a twin to operate over London.
Al