To small maybe to realistically fight Colorado's or later SD/No13/G3s etc, but with hindsight 13.5" isn't really to small to fight German 11", Italian 12.6" or French 330/340mm. I would even suggest that its not totally outclassed by IJN or USN 14"...
The problem is that the layout of the Orion/KGV/Iron Dukes wasn't exactly what you could call great. They had all seen hard long service in the war, steamed a great many miles. Their Q turret had a limited arc, and it made a mess of the layouf internally (its why it wasn't repeated in later ships). Also their armour scheme was seen as being a bit obsolete and their torpedo defences was lacking. Without either ripping out the Q turret and then adding more machinery, you'd also have to lengthen their hulls as you can only do so much with raw horsepower. You've then got to beef up their deck armour, improve torpedo protection etc etc etc and it all gets very very expensive. Perhaps the RN could have built a 'cheap' battlecruiser using the 13.5 gun in say 3 triple mounts but basically the older 13.5-inch gunned ships were not worth the money needed to upgrade them. The only one that was, is the Tiger and she's a popular subject of WI's about her being retained.