The issue of single crew has nothing to do with grandfather rights. Provided an aircraft is certified for single crew operation then you can operate it thus. A surprising number of surprisingly complex aircraft ARE certified single crew (some Citations and King Airs for example), but some insurance companies insist upon two crew. The Trislander/Islander fleet are certified for single crew.
For the Trislander and Islander, there is little that a second crew member could do other than reduce payload. In fact, some of the Aurigny Trislanders do not have a second set of controls. Single pilot Public Transport operations do have to comply with slightly higher weather minima because of the lack of a second crew member looking out of the window!
The main issue is that of an autopilot. The law requires that an aircraft operated for Public Transport under Instrument Flight Rules (as the Rockhopper and Aurigny ones are) must have a serviceable autopilot to be operated single crew. Aurigny and Rockhopper do not in most cases have an autopilot, and it is perhaps this exemption which can come under the auspices of 'Grandfather Rights'. I suppose the argument is that they have operated for so long without an incident attributable to the lack of an autopilot that the CAA is prepared to continue in this way. When Rockhopper started, they could not insist upon them having an autopilot as that is unfair, nor were they keen to insist that Aurigny had them fitted.
The only limitation is that they cannot operate into the major London Airports or Manchester without either a second pilot or an autopilot.
For the benefit of 101's friend, I can heartily recommend this way of going. The pilot has a more thorough medical check up every year than the average bus or truck driver or motorist with whom you share the roads, is regularly checked for competence, and has a vested interest in surviving the flight as well!! Everyone gets a window seat and from 3-5000 feet on a clear day the view is amazing. On Rockhopper you will be offered earplugs... a good idea, especially if you are the back of the Trislander.
