Robin
As we both know British Police Forces have always been slow in equipping Officers with adequate protective clothing, remember seeing dustbin lids, short wooden truncheons and tall helmets in the late 70s early 80s in riot situations! I would imagine things were much the same in 1926, perhaps your Grandfather found his old helmet and thought 'that would be useful'!
Even if military style helmets were officially issued I doubt it would have been the rare 'Cruise' helmet.
Simon