The Japanese "13 Class" ships are often called either battleships or battlecruisers. In the "8-8 Plan", they were classed as battlecruisers. They were very long and narrow, with dimensions of 900ft x 101ft x 32ft. The armament was 8-18/45, 16-5.5 QF and 8-4.7 AA guns. They were also armed with 8-24 inch TT, mounted above water. The belt was to be 13in and the deck was 5in. They also had a maximum speed of 30 knots.
For a British analog to the "13 Class", I would use dimensions of 900ft x 108ft x 32ft. The main armament would be 8-18in/45 guns. The British, in 1921, wanted to push limits of the displacement, while going with what they thought was leading-edge design principles, so they would have used triple 18in turrets and twin secondary turrets. I will stay close to the "13 Class" and use twin 18in turrets and 6in QF guns in casemates. That would give an armament of 8-18in, 14-6in QF guns. I would try using a 13in armour basis for protection, with a 5in deck. The 30-knot speed would be adequate, so I would retain that, as well. I will be interested to see how great the displacement ends up being.