Atco-matic and hydroforming bushmaster - the reason the dash does'nt go above FL090 between JER-SOU or even GCI-SOU is that it is at the request of airtraffic - both the channel island's and SOU have a agreement in place which enables operator's to depart and run the tight schedules they have between these destinations as long as they dont go above FL100 - this removes the a/c from entering the airway system and being subjected to lengthy ATC restrictions(slots). Which can be horrendous through the Hurn sector with traffic passing through here from LHR and LGW.
Hope this answers your querry.