To be fair, I'm not sure if there are actually 2 x 20kg bags of magazines plus other junk - my comment above may have taken advantage of a bit of artistic licence, as the OP seems to suggest they'll just have the two heavy bags.

Perhaps I'm a bit mad, but I can see the via St Pancras option being kind of workable, at least to someone who's prepared to go through a bit of hassle - and the OP appears to be such a person (they made the via Victoria and two Tube lines route work after all!). Wouldn't recommend it during peak times when the Piccadilly line is sardines though...

All that said, given the circumstances, the National Express coach does seem very much like the obvious way to go