AFAIK in order to get a full JAA ATPL you need amongst other things 1,500 hours total time, 500 of which are on a multi crew aircraft.
Now if the UK CAA has as part of a UK operators AOC stipulated that the Kingair must be flown as a two crew aircraft (whereas I think the manufacturer said it can be flown single pilot) then the 500 hours on that aircraft can count towards the issue of your ATPL. If the AOC says that it can be flown single pilot BUT the operators elects to fly it with 2 crew then I don't think the time spent as safety pilot in the RHS can be logged.
There are other requirements for ATPL issue as well which are all laid down in LASORS eg: number of night and PICUS hours etc etc.