'The residue of discretionary or arbitrary authority, which at any given time is legally left in the hands of the Crown.'
'It can only be applied to those rights and capacities which the King enjoys alone, in contradistinction to the others, and not those he enjoys in common with any of his subjects.'
'Does it produce a legal effect at common law and is it unique to the Crown and not shared with any other persons.'
'The prerogative is really a relic of a past age, not lost by disuse, but only available for a case not covered by statute.'
If something is covered by statute and the prerogative it must be done by statute.
If the statute does not grant a monopoly over the particular power over which the executive is relying on the prerogative and the prerogative power exists, then the executive can use the prerogative despite the presence of a statute regulating the relevant area.
Prerogative powers cannot be exercised to frustrate the will of Parliament as expressed in a statute.