Most likely planes leaving the Biggin Hill or Ockham stacks to approach Heathrow for a westerly landing.
Biggin Hill is in SE London and Ockham is to the SW.
Planes funnel into a single path between the two stacks and then fly a 'S' shape to align with the westerly runways.
The second turn in the S would place the planes on a northerly heading approximately in line with the Thames as it runs past Houses of Parliament, St Thomas's Hospital & Millbank.
For visibility the planes would have their landing lights on.
IMO these are probably the bright white lights reported.
https://assets.heathrowconsultation.com/wp-content/uploads/2018/01/2887-HRW-3R-AIRSPACE-Operations-Handbook-A4_V11.1.pdf