I think the biggest step to getting a low-cost carrier would be improving the landing aids to Cat III.....if that was possible with the terrain. With the weather record of the channel islands, any low-cost carrier who attempted to run a robust schedule through there, given the tight flying lines required to achieve the aircraft utilisation, would be crazy given the amount of weather disruption.
"Traditional" airlines with an aircraft planned to operate LON-GCI-LON-GCI... all day can more afford to take the hit.......a few hours of weather delays means most of the businessmen can walk away and plan their meeting for another day, and the load factors mean you are in a reasonable position to consolidate flights once the weather does clear. Those same economics wouldn't apply to a low-cost through GCI.