Whilst many of the non-capital groups don't actually work out any cheaper than renting an aeroplane, they offer two distinct advantages:
1) No daily minimums. You could take an aeroplane and go to Jersey for a week and just pay the hour there and the our back (or so).
2) Aircraft tend to be better equipped, with IFR certified avionics and GPS's etc...
If you paid £110 per month standing charge and then only did 25 hours per year, that increases the cost per hour to £112 per hour, which still isn't a bad rate for a nice aeroplane. However if you drop to 10 hours per year, the hourly rate goes up to £192 per hour....something to bear in mind.
The Twin Star group looks fun...I wonder where it is based? I have just come back from renting one in the USA which worked out in todays money of £115 per hour. The one on that group works out at £140 per hour for 50 hours per year which seems like quite a good deal.......