I usually just walk from King's Cross to Euston, but that's dependent on weather and luggage (takes ~10 minutes), and because I feel like spending a few minutes outside!

If this trip is going to take place around Christmas, then that's information that would have been helpful to know earlier.
e.g., there are no time restrictions on the off-peak rail tickets between 22nd December (pm) and 2nd January, and some trains will be very busy and a reservation may be advisable.

(the Swissport strike is off, and BA say all flights will be running)