Buying train tickets when not sure of arrival to King's Cross Station

I need to correct some inaccuracies in PP, some details were surprising to me. If you plane is on time, it would take less than an hour to clear immigrations and find the train station in the airport. Regular readers here will know that I don't normally fly but I have picked up more than more than my fair share of people there. I've never seen them come out inside an hour, often over 90 minutes. An hour and half with couple of changes to get to King's Cross via the tube.
54 minutes, 24 stops, zero changes from Heathrow T3 to Kings Cross St Pancras via the Piccadilly Line. Thirty minutes to Paddington. The Heathrow Connect option does in fact take 30 minutes, the Heathrow Express takes 15, from Heathrow to Paddington, previously noted that connection is still needed to Kings Cross, neither covered by Oyster or Travelcard. Kings Crossing I'm really sorry to bring it up, but there is no such place. Its Kings Cross. Kings Cross rail station and St Pancras rail station are connected by Kings Cross St Pancras Underground Station. There is no Kings Cross ing. I think you could safely buy a ticket for after 1100 AM or noon. The train up North can be a bit tricky on that day. What you really want is a Goldilocks train. The available trains unfortunately are either too hot or too col. I'll continue with the three choices in the next post as there isn't enough space remaining in this one. (to be continued)

(continued) There are three train choices, none perfect. The latest one, and best if your connection is a little after 1500 or flexible, assuming midweek travel in the next couple of months is the 12:30 East Coast service to Newcastle calling at Peterborough, 13:15, Newark North Gate, 13:44, Doncaster, 14:09, York, 14:35,
and arrives Northallerton at 14:54 only 6 minutes before 15:00, total duration for that trip 2h 24m. The next earlier leaves at 11:35 and arrives at 14:18, better for the connection to the Dales, but it involves two changes at Doncaster and York, and is more expensive. Earlier than that, by a few minutes, is the 11:23 Grand Central Railway service to Sunderland calling only at York at 13:24 and Northallerton at 13:44 (duration 2:21). That leaves you quite a lot of time at Northallerton. Grand Central is very much a minnow among British railway operators about which I know virtually nothing. So you really need to think which bowl of porridge is best. IMO, if your plane is on time at 0800, you may well be on the Piccadilly tube by 09:30, so into Kings Cross before 10:30. Even if your flight is nearly an hour late you should be nearly OK for the earliest bowl of porridge. Do be sure to leave time to figure out the station at Kings Cross and collect or buy your ticket. Happy trails...

Ah, the PP to which I was responding, has poofed.

Re Cynthia's suggestion. You could also use any extra tiime just walking around London. Piccadilly, Regents Street, Bond street is a delightful way to spend an hour or five.

One quick warning about Kings Cross/St Pancras and the British Library: the BL now no longer allows any large luggage (more than a budget airline hand luggage) into the building because they were fed up with cheap tourists using them for luggage storage and not leaving any cloakroom space for their proper users.