I think it is a pretty sound business model for the US but am not sure it will transfer readily to Europe and the UK in particular. For a start the US has a much bigger population on which to draw. The UK has a much lower population as we all know and it will cost dearly to try and reach your target market. Look at why most airlines fail and principally no one knew who they were or what they are delivering ie: poor marketing.
Other reasons conspiring against air taxi are:
ATC/Airspace - to make a go of it you want to be able to get up and go when you want with minimum of fuss. Anyone who has flown in the US knows how user friendly the airspace is and there is no shortage of airports with 10,000ft runways and decent terminal areas which you can land on for free. Contrast that with the UK's airspace/airports and the need to file IFR flight plans and pay Eurocontrol for the priviledge. I can see the short notice being a real issue for filing flight plans and getting around flow controls etc. To make air taxi really work you want to be able to take off VFR and if necessary file IFR in the air or better still go VFR all the way if CAVOK and stay under busy airways and utilise smaller airports but I am pretty sure there is something somewhere that will stop you from doing this.
CAA/AOC - If you go for one of these new fangled light jets then there is the whole JAA certification issue to get around. Will the manufacturers be willing to pay for this when presumably they won't be selling that many units in the UK? Answer - probably not, so the operator might have to chip in. So that pushes you towards something established like the beech 1900 etc. Then there is the issue of an AOC which in itself is a relatively expensive and time consuming document to get approved. Yes you could try piggy backing off someone elses for a while but there is no such thing as a free lunch so expect a major share of any profit to be deducted.
I like the concept a lot however I can see just so many hurdles against it.