Weather is the big factor with the mainland to Scillies transfer - Lands End is up on a hill and Penzance has no instrument approach at the mainland end and at St Marys you can't get below 860' QNH on the RNP approaches to an airfield that is 357' amsl.
Tresco at SL would be better but there are no instrument procedures there.
Advection fog is the big enemy in the SW. Holding div fuel just means fewer pax and less revenue.
Both St Marys and PZ used to have an NDB. L-End (400ft) had the Cot Valley and red & white telephone poles for the Rapides !!!
St Marys (116ft) and Tresco PZ (sea level) were not usually the problem and the S61 had a suitable Radar to help. I seem to recall that an NDB app got you down to 600 ft on Scilly so no point in going above 500 on a transit if it was marginal. L-End is not suitable for serious instrument approaches due terrain and lack of approach lighting. The new GPS approaches do not get you down soon enough to see the field in time to land if marginal. BEA could not leave L/End soon enough when the sea level factor and S61 became available. The 'bus like' regular and reliable service this offered changed the way people looked at air travel to Scilly and it certainly set the standards for a 'transport system' coping with the local weather conditions. EB had been designed for quick turnarounds and RR refuelling but was always reliant for Culdrose or St Mawgan for a diversion.