ATP Al,
In the old days one could fly LON/ISC/LON with a connection via PLH, one could route from anywhere to LHR and/or LGW to fly to ISC via PLH ... and then a BRS/ISC service was added to serve those driving down the M5/M6 side of the country who didn't fancy the summer congestion of the Devon/Cornwall roads.
All that went when Brymon got rid of the Twotters. Since then PLH has gone, Land's End is not suitable, I don't think EXT would work because would need to carry round trip fuel EXT/ISC/EXT but then NQY has up country routes, all it might take is a BH2/DHC6 operator to set up in NQY and fix a schedule around the arriving and departing up country routes.
That said Brymon didn't make, indeed lose, their money on a sole ISC operation and I think any incoming operator might struggle to make ISC profitable on an all year basis.