What large armoured- and protected-cruiser fleets taught Britain

What did Britain's large fleets of armoured and protected cruisers teach the navy?

They demonstrated the need for numbers, while each successive class carried lessons from its predecessors.

The broad lesson offered for both cruiser categories is the need for sufficient ships. Individual classes also incorporated the successes and failures of earlier designs. A detailed answer would have to distinguish those class histories rather than assign one technical lesson to the whole fleet.