If I remember correctly, the RN had to try to juggle with a number of requirements. Namely, complying with treaties / limitations, having sufficient cruisers to have a presence around the Empire and secure trade routes etc. This meant that these cruisers needed to be quite long legged, have good speed and also have a good combination of armour and firepower to look after themselves - as backup may be a long way off!
I believe the 8 inch goods whilst quite hard hitting, had a slower rate of fire and so were second choice to the more rapid firing 6 inch guns.
The IJN tried a different route - emphasizing firepower over protection and creature comforts for their crews. In some cases the IJN produced ships that were not very seaworthy and had top heavy characteristics.