I think price is the only thing that gives the SEN route any chance of success.
There appears to be only one train that will get you into London before 10am and it costs over £200. Based on MAN fares a last minute ticket to London via SEN should be around £150 flight + train and really you should get into London by about 9am.
I have my doubts as to whether many people will use the early flight heading north, I also wonder if people will fly down and just get a train back later on.