There are a number of routes and airports in the British Isles that could be served by a respectable operator of small planes
Exactly and that is why reproduction of, otherwise soon to become extinct, such types as DHC6, Do228 and Do328 have recommenced.
Going back some 6 years a (rich) friend of mine was thinking of starting a 'puddle jumper' airline with a fleet of perhaps Do228's & Do328's, at that time there were petition(s) to keep Plymouth Airport open so that's how he became involved and interested.
But besides Loganair, Iscles of Scilly Skybus and LyddAir there are no 'puddle jumper' operators remaining whilst there are IOM routes suitable for such types, apparently Gloucester was successful, Belfast and Glasgow, Blackpool and, from what I've been reading, if such an operator can undercut the extortionate fares that Eastern want for a 20 minute flight to Belfast then they'd be on to a winner!
But an IOM based operator probably wouldn't work nor would it likely utilise more than perhaps two aircraft, when were friend and I were looking at it our plan was to base such an operator in Wales (Cardiff) for the government handouts.
But there are plenty of other airports out there where 'puddle jumper' services would likely work and all could be achieved by operating 'W' pattens from a main operating base or two:
Gloucester/Channel Islands (summer only) has been going since DC3's operated the route.
Channel Islands/Oxford (summer only) may work.
Waterford, ATR's & Fokkers proved to provide excessive capacity but a route or few available there for a puddle jumper.
Kerry ... Pretty much ditto as I understand it.
Belfast to Cork seemed to be viable in the past.
Cardiff are trying to develop a route network but not so many routes can fill a Q400 or similar.
Newquay ditto
And if they were to reopen Plymouth Airport then a number of routes to be served from there.
The list goes on.
In the old days business routes were operated, at least, morning and evening whilst leisure routes filled up the gap(s) in between, these days "We have a business route but you need to travel at 6am but only on Tuesdays, Thursdays and Sundays"
That's no good and in time someone may come to realise that it's not all about "Let's squeeze as many bums on seats in as short a time as possible to satisfy the accountants"