Only hope to do something sensible is to extend the runway into the island (forget going out into the sea - phenominally expensive) - 600m simply is not enough for anything useful other than the Twin Otter.
However, who would pay? - is this part of the Cornwall Council portfolio or is it Duchy of Cornwall-owned land - i.e. are we going to ask Prince Charles to stump up the £1.5 to £2m or so costs to just add 300 or so metres to the runway? Who owns/leases those fields to the north and would the local authority move the road at the end to squeeze an extra 60 or so metres more? Could they get European EEC funding for such a prject, or at least a contribution?
To the first road you'll extend to about 800m licensed length, to the second road about 850m - depends on Runway End Safety Areas (RESA) deemed necessary. Read somewhere the land dips notably inland though so additional earthworks will ramp costs up significantly.
That just about brings it into the realms of a practical Dash-8-100 commercial operation - with limitations on passenger load/range.