2 types: High frequency ATRs to the mainland, plus C208B's inter island and ACI-SOU is the solution. The latter can be done with careful routing, altitude and synthetic vision options. 1 P&W PT6 is far more favourable than 2/3 piston engines: locals have idolised the BN2/3 with single crew for 30 years, so single point of failure has been the choice for a generation. And I'd choose 1 PT6 over a fully laden piston variant every time. Better still, 1 PT6 and 2 crew.
IFSD rate is quoted as 1 per 12.4m flight hours.
Are you suggesting they drop the Gatwick connection? 5 times 75 does not come near the daily requirement, can you magic up the extra slots at Gatters which are not available? High frequency ATRs would be nice but the Gatwick end won't allow them consequently a larger plane like an E135/ B737 is needed..........back to the current number of types mix.