Why favour the F3 battlecruiser design?

Why choose F3 instead of the Nelson and Rodney design?

The proposed F3 offers full armour, nine 15-inch/50 guns and more than 28 knots, a combination judged more useful for a later war.

The follow-up asks why F3 would be preferable to the Nelson and Rodney design. The proposed attraction is a fully armoured battlecruiser with nine 15-inch/50 guns and a speed above 28 knots. The preference rests on combining heavy armament with more useful speed for a later war, while still acknowledging the value of Nelson and Rodney. This is a design preference inside the time-travel scenario.