Edinburgh to London with a stop in York

You can do a walk up but buying in advance saves money.

Is the trip from Edinburgh to York and London a trip on its own or the return leg of a London > Edinburgh?

If your trip is a return, a break of journey on the return leg is often permitted. This would not be the case with "Advance" tickets which are generally non-changeable or changeable with a significant cost and only before the journey commences.

Would you like me to check on anything for you?

The Edinburgh to London by way of York is a the last leg of our 3 week trip around England and Scotland. Up until that point we will have a car but will return it in Edinburgh. We fly out of London to return home (and also seeing a concert there before we leave).
I figure the walk up ticket will be more expensive- but wondering if being a little more open on the time we leave York for London will be worth the cost or if we should just pick a time to return and call it good.

You can look at fares on nationalrail.co.uk for same-day travel now (especially on the same day of the week) and compared those to what you see for your planned travel date in late June. Is the difference enough to matter to you?

York could keep you busy for more than one full day, so you probably don't have anything to lose by picking the latest train you think will be reasonable for getting settled into your London hotel that night. It's not as if you're going to decide in the middle of the afternoon that you have run out of things to do in York.

since this isn't the return leg of a period "return" you will be well served by making a plan and sticking to it. The walk up fare York to Kings Cross is something like £123 per person. You can do ever so much better working out a lateish fixed time and getting a previously booked much much lower Advance ticket for each half.

on the other hand, we don't know anything about your priorities or financial situation. If whimsy is more important than money in this situation, by all means go with the walk up fare in York. You'll be able to get on just about any train, and there are very frequent fast trains to London. Just pay the money and hop on...

Ha! Yes, one does have to weigh the cost vs. the importance and fun-- and sometimes just go for it! Thanks for the insights, all.