As an outsider, it seems to me that it is just under 30 miles from Lands End airport to the Isles of Scilly airport. I also run a company that offers a similar service from Long Beach, (our private helipad at the Queen Mary), to Santa Catalina Island, which is just under 30 miles offshore.
Island Helicopters Not trying to advertise---feel free to remove link if deemed not appropriate.
We are using Astars, a Jet Ranger and an S-76, and charge about the same as your Islander flights cost, ($250 round trip). We schedule flights as needed, obviously in the winter we are slower, but in summer, we will have 3 aircraft doing about 10 to 12 round trips per day.
Instead of trying to go big, why not start small and build up..... just an observation is all.