Your characterisations of the RN cruisers are a bit off:
The Leander class did not have "destroyer guns", but rather the usual light cruiser 6" (15,2 cm).
The Leander is the archtypical light cruiser, after which pretty much every other nations 1920s-30s cruiser (except for the US and Japan) was modelled until the japanese kicked off the "heavy light cruiser" race.
County class cruisers were neither slow nor heavily armoured.
Cumberland had a heavier belt armour than Graf Spee, but not much, and was faster than Graf Spee.
You can't really say she was "old" (commissioned 1928) in a navy that still had that many WW1 vintage ships at the time.
ALL RN ships at River plate were faster than Graf Spee, at least in Sprints.
The great advantage of the pocket battleships was not their sprint speed, but their sustained speed and fuel economy.
A pocket battleship could operate at a sustained speed of over 20 knots for weeks without refueling.
__________________
|