Was watching on Planeplotter last night and a few went into Stansted late on (11ish).
As to why LGW was closed and LHR not, I am sure it has to do with the fact that LGW has only one runway, is probably higher (??) and I think that the south end of London and into the Sussex seemed to be more in the line of the snow.
Luckily we have no major airports near me (in Derbyshire) because we are seeing more snow then I have ever seen. We currently have 40 cm of lying snow with more predicted.
Cheers, Andy S