The National Express ticket machines at Kings Cross have been altered
to sell the correct versions of some tickets for journeys starting at
other London terminals. They now offer tickets via Waterloo or
Paddington with the U1 journey included. Tickets to Thameslink
destinations are available stating From St Pancras not via
Underground, and also to some SWT destinations stating via Wimbledon
not via Underground. I always thought that London Underground didn't
like National Rail selling tickets for journeys that started on the
tube. I also wonder why it's only Waterloo and Paddington that are
included.