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The WHO link you added lists different methods of insecticide application:
The different procedures currently in use are as follows:
treatment of the interior of the aircraft using a quick-acting insecticide spray immediately before take-off, with the passengers on board;
treatment of the interior of the aircraft on the ground before passengers come on board, using a residual-insecticide aerosol, plus additional in-flight treatment with a quick-acting spray shortly before landing; and
regular application of a residual insecticide to all internal surfaces of the aircraft, except those in food preparation areas.
It sounds like the process followed on your flight was the second one, and the spray you saw was the in-flight treatment before landing. The recommended procedures for this are described further in this report (for example, it notes that the overhead bins must remain open, which is not generally followed in my experience). Most of the flights I've taken where insecticide was used have used the first process.
The US Department of Transportation lists disinsection as voluntary for flights to the UK (it's unclear to me if the voluntary nature is true for both malarial countries or just Zika countries, and I couldn't find a UK government source), so it may be up to the airline what method to use.