Seat reservation question

The train is a mix of reserved and unreserved seats. You pay extra for reserve seat. You get on and find an open seat and sit down. If traveling A to D, you may find that a seat is reserved only B to C, or A to C. It just depends. You may have an unreserved seat for a couple of stops and then reserved so you have to move. For long trips a reserved seat has an advantage.

The train should start in Edinburgh, so it shouldn't be a problem getting a seat.

When are you travelling and what is the ticket called? Is it an Advance ticket, Off-peak, Super-off peak or Anytime ticket?

An Advance ticket will include a free seat reservation. With the other ticket types you need to find an unreserved seat. If there are no unreserved seats you stand.

If you need to reserve a seat you should do this at the station no later than two hours before the train starts its journey, or, for early morning trains, up to 1600 the previous evening.

If you have bought an Off-peak, Super-off peak or Anytime ticket you might consider applying for a full refund and buying a much cheaper Advance ticket from the train operator when they go on sale (about 12 weeks before your travel date).
https://www.lner.co.uk/

If you’d booked with the train company you’d have been offered the opportunity to reserve a seat on a particular train as part of your ticket price, even if you were buying an anytime or off-peak ticket.

But don’t worry - there are unreserved seats. There’s often a carriage that’s entirely unreserved, and staff at the station will tell you which one. Also: there’ll be some unused reserved seats - you’ll find these once the train has left the destination. Give it a while, and then if there’s a seat that says “reserved from Edinburgh Waverley to...” that doesn’t yet have anyone sitting in it, feel free to take it (on condition that you move if the reservation-holder turns up).

If the train is super-busy, you can sit or stand in the corridors until a seat becomes available. The train guards are good about making announcements about where there are available seats.

What ticket type have you booked? Sounds like you booked a flexible off-peak type, that's why I don't understand the seat reservation option not being given.

An Advance ticket type (i.e. discounted, non flexible, specific train) automatically gives you a seat reservation, the Train company (LNER) online page should have taken you straight to this option, plus further preferences of aisle/window/ forward/backward facing/quiet coach/table etc,

I did this journey recently, at Waverley station, the platform doesn't show until 15-20 mins before departure, then a large throng of passengers move swiftly to the train with their bags, , nobody guides you on, if reserved you find your coach and seat number, if not reserved you take a chance about standing, the fastest journey time is 4.5 hours, I'd want to know I had a seat with my further preferences.

Loco2 asks whether you want a seat reservation if you are booking a non-Advance type ticket, which is a pretty standard thing today. If you didn't check the box you wouldn't have been allocated one.

You can get a reservation at stations, and usually by calling the train operator (LNER in this case), or even sometimes via email or Twitter.

Thank you all for your help. On closer review, I now see that I mistakenly chose an off peak ticket. I’ve now rebooked and purchased an advance ticket, and as you said it comes with reserved seat and is cheaper. I’ll apply for refund on the off peak one. Thanks!