David A Stocks wrote:

It's possible the following condition applies for the Heathrow to King's
Cross part of the journey:

"Off-Peak tickets for journeys
starting within the Network
Area to destinations outside
the Network Area with travel
via London are valid by any
train to London, to connect
with trains from London."

That's talking about off-peak tickets, not railcard-discounted tickets,
so isn't directly relevent here. I think (could be wrong) that
restrictions on off-peak tickets generally *are* taken with respect to
the National Rail portion of the journey.

The reason that I'm less sure about the railcard case is that you get
the railcard discount on the entire value of the ticket, not just the
national rail portion.

Be aware that if it doesn't the ticket could be rejected by the gates -
although if you have luggage you may well be using a manual side gate.

Actually, I very much doubt that the LUL gates are able to make such
subtle distinctions.

-roy