dooy,
The Twotter cruises at 150kt which is bl00dy slow due much due to it having a fixed undercarriage, the Twotter is also only favourable for short strip, water, ice/snow operations where other types may not operate.
The DHC7 was Canadian built primarily for the Canadian market, many Canadian airfields/airports are/were short (ish) gravel (or similar) strips, DHC products were in their element in Canada, operators could operate far and wide short strip operations utilising the aircraft dusk till dawn daily.
DHC products only became present in SW England due to PLH, ISC and LEQ and their short and/or grass runways and one isn't going to utilise a 50 seater STOL airliner dusk till dawn 6 or 7 days a week on an ISC route network!
Another problem on ISC is the lack of fuelling facilities, apparently Skybus have their own limited supply on the island, I guess for unforeseen circumstances, but without a common fuel supply there aircraft need to tanker in the fuel for their outward legs thus limiting their range and limiting any ISC route network.