Keith, you're absolutely right, even a sailor (or a scout) would have trouble recognising a flapping flag as being upside down. But if you hoist an ensign upside down, it's perfectly obvious, since the Union is at the bottom instead of the top. (For non-UK Catters: the red white and blue ensigns consist of a flag of that colour (with a St George's cross in the case of the white) with a Union flag in the top quarter nearer to the flagpole.)

The only question then is, how do you attach it to the flag halyard, since the top and bottom attachments are different to stop you doing just that?