Technically, DSA has a domestic service to Jersey
I doubt a London service would work because of the frequent train service, except for maybe LHR, which isnt going to happen any time soon due to slots
I think EDI and GLA could work, and especially Belfast. Flybe would be ideal but they have a 'base' at LBA. Thye may also start HUY services in the future. They could always serve DSA instead of HUY.
As for Belfast, maybe an option for TOM. Wasn't Belfast on a list of possible destinations released by TOM in 2004 when they announced their DSA base?