Jabird, for those of us in West Middlesex, SOU is definitely much more of a "London" airport than say LTN or STN, certainly in terms of proximity and accessibility.
Rubbish Dump Interchange (sic) is just one example: HS2 in London will be at Euston not St Pancras, so no link to HS1, at Birmingham itwill be at Curzon Street so no access to the New Street interchange, HS1 at Stratford is nowhere near the existing Stratford interchange.
We never use railway interchanges to their best potential - there are countless examples, and that pushes people off rail and onto the roads.
There's no joined up thinking on the railways, so don't expect it in aviation. So in the unlikely event of SILVERing Sands ever being built, DO NOT expect any associated infrastructure to accompany it!