England's safest pair of hands, perhaps, to trust when filling gin? (6,5)

'england's safest pair of hands perhaps to trust' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'when filling gin?' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?