If I remember these ancient aircraft correctly, the Whitley flew with a nose down attitude, so may have been perfect for scanning the sea!. As Reg has already said, the .303 calibre Browning was fairly useless against aircraft, so bullets would have just bounced off a U boat!!
Principal reason would be to force it to dive, and thereby use its slower electric motors and thereby lose contact with any shipping it might be trying to attack. It could also alert RN ships to the position to hopefully make a successful attack, while the merchantmen carried on. Firing might have "encouraged" any sailors that diving was a better option than staying on the surface to fight (sadly when they did, inevitably the RAF came off worst).