I don't have a problem with the Islander for the job. Many of the runway's are under 450 or 500m, with one only 381m.
A possible replacement would be a Cessna Caravan. The short fuselage version with the higher HP engine. The catch is the 381 m runway. A C208 *might* be able to do it with the same load that the BN2 currently does however Cessna's figures require extrapolation to determine the case. Even Cessna's UK agent was unable to supply manufacturer data for the case.
There's still the anti-SE turbine mindset endemic in the CAA & many operator's management that would have to be changed. The UK authorities also seem to have an irrational fear of GPS & GPS approaches even though many (most?) of the airstrips don't have navaids.