Passengers with layovers in London will normally have their luggage sent to their final destination from their original airport of departure. This will however depend on whether your flights into and out of London are from the same airport:
London Layovers: Arriving and Departing from the SAME Airport
Passengers on layover tours arriving and departing from the same London airport will normally have their luggage sent to their final destination from their airport of origin. Passengers are welcome to bring their carry on hand luggage on board the tour.
For example:
A layover in London in between flights from Dallas Fortworth, Texas, USA to Johannesburg, South Africa connecting at London Heathrow. Luggage will be sent directly from Dallas to Johannesburg.