For pretty much every RAF bomber (Lancaster/Manchester, Halifax, Wellington, Whitley), the nose gunner was not a dedicated position but a task that the bombardier performed when not on the actual bombing run. It was an efficient use of personnel because the bombardier was only needed during the run over the target when, for the most part, the biggest threat was from flak rather than night fighters. For the rest of the mission, he'd just be deadweight so giving him the nose gunner job actually reduced the number of personnel required to man the bomber fleet.
Outside of the bombing run, having an extra pair of eyes in the forward hemisphere was a valuable addition to enhance situational awareness for the pilot. As others have noted, this function often had more to do with avoiding collision with other aircraft in the friendly bomber stream than it did to defend against enemy night fighters.
The removal of front turrets later in the war was largely to reduce weight and drag to improve speed. It did not result in smaller crews, however, because there was still a need for the bombardier.