There are regarded as "discrete plates" and the procedures can only be legally flown by approved operators - eg. Loganair, GAMA to name but two.
The reason for this is that full ATC is not present. A FISO service is provided by HIAL to reduce the costs. This lower type of service is not permitted to separate IFR flights - only pass traffic information.
The theory is that only one IFR aircraft will be in the system at once....." I have no known traffic to affect"........"land ~ take-off at your discretion....."
CAP 413 will give you the R/T variations to normal ATC.