London to Leeds by train

Final Destination: Leeds (or Bradford, or Harrogate or Skipton)

All trains also call at: Wakefield

If you want to travel at the cheapest possible price, look for the 'Advance tickets', though the two key things worth knowing about booking and using 'Advance' tickets are:
(1) This type of ticket can't be refunded if you subsequently change your travel plans, or miss the train in circumstances not to do with a connecting train.
(2) A £10 admin fee will be payable if you want to exchange your ticket to a different departure to the same destination, to that which you selected when making your booking PLUS you will ALSO be charged any price difference with the new ticket you'll then have to purchase.
(Though the T&Cs of using Advance tickets have been made more flexible for journeys taken before November 30th).

What is unusual about booking with LNER is that Advance tickets are now available online up to only 5 mins before departure.
They will have sold out sooner on the most popular trains, and the earlier you can book the cheaper they will be, but before buying walk-up Off Peak or Anytime tickets at a station, just prior to departure, it's worth checking on your phone to see whether Advance tickets are still available, before heading to the ticket counter or machines.

On the least busy Monday to Friday departures 'Super Off-Peak; tickets are usually available, they'll be more expensive than Advance tickets, but will give you greater flexibility with your travel-plans.
You will only have to pay any price difference with the new ticket; if you swap them to a different departure more than  24 hours ahead, there will usually be no difference in price to pay.
They can also be refunded for an admin fee of £10.

Travelling on Monday - Friday:

Tickets are usually available 12 weeks ahead of the travel date.

However, when you look up a journey you may see dates further ahead on the calendar, but you'll be informed that tickets aren't yet available for those dates.
On the LNER website you'll be prompted to sign up to a Ticket Alert service, on a PC, over on the right on the screen you'll be taken to, you'll see the furthest date ahead that tickets can be booked for.

Travelling on Saturday-Sunday

Tickets are usually available 12 weeks ahead of the travel date, but maintenance work on the route is periodically undertaken at weekends and if it is scheduled on your travel date, it can affect when tickets will be released for sale.

If you're looking up a journey less than 12 weeks ahead and tickets aren't available, it can be a good indication that works will be impacting on your travel date.
It can be worth checking this on the National Rail website - enter LNER as the operator.

Making an end-to-end journey will still be possible, so it can be worth signing up to LNER's ticket alert service, though you MAY ultimately have to take a substitution bus service for part of the trip.
It's the confirmation of these alternative travel arrangements which can hold up the release of the tickets for sale.

Choosing a departure

There can be significant differences in prices between departures on your travel date, it's possible to save more than £30 by taking an earlier or later train.
The LNER website makes it clear what the cheapest price is available per day.
When heading north, the trains which depart London in the middle of the day tend to be cheaper.

Reservations:

When you book an Advance ticket for a London to Leeds train journey, you will be automatically assigned a seat(s), but if you book the other types of ticket ahead, you can request a complimentary reservation - more info on this

Seat reservations aren't automatically included when booking Off Peak or Anytime tickets online, but you can add the complimentary reservation, once you've made choice of departure, either when booking or afterwards.
When booking walk-up tickets at a station ticket counter, if the Advance tickets aren't available for the next departure, confirm whether a seat reservation has been added to the booking of an Anytime or Off Peak ticket, in theory reservations will be available until around 5 mins pre-departure.

If you will be using a rail pass, including a Britrail or Eurail pass, you should be able to obtain a reservation, at no charge, from a ticket counter up until 5 mins before the departure; keep this in mind as it is not the norm when using rail passes in Britain.

Receiving Your Ticket(s)

If you book with LNER the four options for receiving your ticket(s) are:

(1) You will receive a booking reference number on your order confirmation email and you can use this reference number to collect ticket(s) from a ticket machine.
Some stations will have dedicated ticket collection machines, but standard ticket machines will also have an option for collecting pre-booked tickets.

You will need to enter your reference number into the machine, so make sure you'll have easy access to it when you are using the machine.
You will also need to insert the specific credit or debit card you used when making the booking, so have that with you too.

Showing the email you have received to staff at the ticket gates won't get you on to the train, nor will this be valid if you can only show the email to the conductor when you are on the train.

If you forget to bring your ticket(s) with you to the station, but do have the card with you that you used when booking, you can use the machines to collect your ticket(s).
Take your time and take care that you have picked up all your ticket(s) and seat reservations.

(2) Print off the tickets you receive before heading to the station - the instructions for doing this are available here.

(3) Opt to pay an additional charge to have your tickets posted to you (not available when booking tickets from outside the UK).

(4) Mobile tickets - if you select this option when making a booking you will need to download the LNER Travel Buddy app so that you can 'Activate' your ticket.
The LNER website states that 'Advance' tickets will be automatically activated, but there's no suggestion on the LNER website that it will be OK to save the ticket(s) you will receive as a PDF to your mobile device, so download the app to be sure that you can use a mobile ticket.