I use Bristol quite a bit - and it's one of Britain's less annoying large airports - and I have to say that I find the bus and taxi arrangements far more friendly and way less perverse than what we have to put up with in Scotland.
- Edinburgh - a busy railway line right at the end of the runway and at the location of the old terminal, rip off tram costs to link from the Gateway station or anywhere else.
- Glasgow - a busy railway line right at the end of the runway but no thought as to how it might be linked. Endless fruitless discussions about a dedicated link.
- Aberdeen - a busy railway line at the wrong side of the runway and at the location of the old terminal. No means of reaching one from the other. Pie in the sky proposals for a dedicated link.
- Inverness - a busy railway line nearby and at least some future plans to connect the two, but a relatively small and little used airport.
- Only Prestwick, Scotland's grottiest and otherwise most useless significant airport, has a railway station. But nobody wants to go there.
So Bristol has some integrity in that it has no nearby rail links to be squandered or abused, or isn't an absurd distance from the city. The buses and taxis are relatively inexpensive and in my experience without exception drivers provide excellent service.
Edinburgh has become a terrible example of what happens when a privately run airport comes into contact with publically funded transport infrastructure and a nearby railway line - it's cheaper for two people to come and go by taxi than it is by tram and train. On the basis of how UK airports are run at the moment, and the evidence found in Scotland, having Bristol closer to a railway would provide few gains I fear.
INV is small but its certainly not little used, considering all its new hub links and that its been running at well over double it's designed terminal capacity for the last 2 years, it get extremely busy in the summer.
Also, any rail link would also be used by the adjacent new town, Tornagrain