Train travel October 17th from Southampton Central to Edinburgh Waverley

Yes, you can't buy train tickets for 17 Oct yet.

What you're also likely seeing is a standard ticket all the way from Southampton to Edinburgh. Search for separate Advance tickets Southampton to London, then London to Edinburgh, and that should be cheaper (when they become available in a few weeks).

EasyJet flies from Southampton to Glasgow - check their fares - I can find one for £48 on 17 Oct. Then all you need is a coach from Glasgow Airport to Glasgow city centre, then to Edinburgh - dead easy. Cheaper, quicker and less hassle than the train via London.

You are too soon, however I can see that there is announced engineering work, not affecting your journey.
The Southampton to Salisbury line has engineering work with replacement buses, also the Bristol to Birmingham route has engineering work with replacement buses south of Worcestershire Parkway. But the Southampton to Birmingham via Reading route (your route) looks to be open
The East Coast is open, having had its annual closures North of Newcastle most weekends in September, with diversions via Carlisle and buses.
However Leeds to York is closed so Cross Country are diverting via Doncaster omitting Leeds.
No word currently on the West Coast north of Birmingham.

The usual advice would be to change to Avanti West Coast at Birmingham if the route turns out to be open.

There should be a 1 change route every hour- generally a change at Birmingham New Street, either via East or West Coast. West Coast is normally faster.

The best price is likely to be via London, with a tube journey between Waterloo and Kings Cross. That would be ticketed (when times are released) as Southampton to London Underground Zone 1, and London Kings Cross to Edinburgh.

I would just fly.

Yes you can probably buy some combination of tickets and passes with our myriadic rail system that comes out cheaper. But it will take you all day.

The 1015 flight lands at midday, and with no immigration control and the efficient tram service you'll be having lunch in the city centre by 1300

there is a problem with the journey, It's Cross Country Trains! Their fleet is not fit for purpose on a journey of this length in standard class - they let me travel for free and I still won't do it! They are also relatively expensive and are holding back on release of the "discounted" Advance tickets - the ones I can see are only about £10 cheaper than what you found and those had some flexibility

I don't know what National Rail are doing cutting off enquiries early because they have another page saying reservations are open much longer https://www.nationalrail.co.uk/travel-information/advance-booking-dates/

If you can manage to change trains in London you can get tickets much cheaper and the ride itself is more comfortable.

Or ask the system for a route via Preston, which will be Cross Country to Birmingham and a more comfortable Avanti to Edinburgh, but that option is only available every two hours.

Enquiries are NOT being cut off early- to be able to offer a through Advance ticket all operators need to have bookings open. So while Cross Country are open longer, neither LNER or Avanti are yet open for that weekend, so you can't yet book from Kings Cross to Edinburgh on either LNER or Lumo, for instance, and can't book a through Cross Country ticket involving other train companies.

Even trying to split fares for previous weekends is not producing anything very useful even just as far as Birmingham, unless on a very early train. I spent a good deal of time on this last night, and nothing is really working.

On 3 October the 0615 and 0715 from Southampton have £19.90 Advance Fares to Birmingham. They arrive at 0851 and 0951.
The 1008 Avanti direct train to Edinburgh on 3 October is £42.40- a split fare on Crewe. So a total of £62.30.

Or the 0715 is £30.70 to Manchester Piccadilly on 3 October, arrive 1129, through train, then the 1224 direct TPE to Edinburgh is £50.90.

I'm on Cross Country today, and was on Tuesday, on the Birmingham to East Anglia service. Different trains and route I know. But perfectly pleasant and uncrowded journeys. I booked 6 weeks ago and got it for under £25 with a railcard each way - from March to West Cumbria. It helps that it is a weekday of course.

On the Cross Country website the £19.90 fare to Birmingham is available on 17 October on the 0515 and 0615 trains, but sold out on the 0715 and all later trains- £94.70 on those later trains.

Thanks to everyone for all the replies, this is all very helpful. I think what we will do on Sat Oct 17 is to leave Southampton on bus or train, go to London, and then from London to Edinburgh. In London I see we would get into Waterloo Station, probably take the tube to Kings Cross Station and catch the train to Edinburgh. I got an email from Trainline saying schedules are out for Oct 17, so that is where I got this route. With senior rail card the discount is pretty good. Havent checked the advanced tickets yet. Does this route I listed here seem workable?

Firstly booking through Trainline will incur booking charges, which won't be the case if you book through one of the train operating companies - doesn't matter which one. And. should anything go wrong, it will be easier to get recompense.

Secondly you say "we". If it's two of you, or any even number, it will be cheaper to get one TwoTogether card rather than two Senior Railcards, unless you are planning separate journeys later in your trip. Both cards cost £35 and give 1/3rd off fares but you only need one TwoTogether Card rather than two Senior Railcards.

Thanks Johnew52. I plan to book and purchase through National Rail and not Trainline, I signed up for the notification with trainline to get an idea when booking would be available. Yes, Its two seniors, wife and I. I will check out the TwoTogether card. I seen that mentioned on the Man in Seat 61 website too.

via London would certainly be a reliably cheap route..

Trainline are right that the schedules are out for the Cross Country trains service, and more digging shows that on their website they have Advance fares from Birmingham to Edinburgh for £42.60- so on Cross Country on the 0615 from SOU the journey can be done for about £62 (or around £41.60 with a railcard), but it is a long roundabout route.

But they are not out for the East Coast main line.

Via London on 3 October (the last Saturday currently on sale by SWR) it is £11.75 Southampton to Kings Cross (including the tube from Waterloo, with railcard), arrive Waterloo 1020, leave KC 1130 £55.30 with railcard- so £67.15 in total

NOTE- You would buy ONE Two Together Railcard for £35, not 2x Senior Railcards at £35 each- https://www.twotogether-railcard.co.uk/

isn31c: National Rail journey planner IS cutting off early for all enquiries, any route/operator, at this moment October 14th is the last you can ask for, presumably tomorrow it will click another day since when the OP raised the problem is was stuck at October13th. That is despite National Rail booking horizons page showing reservations open on several operators well beyond that date and operators own websites selling tickets, even Advance, beyond that date.

Tickets on Cross Country and GWR for 17 October were released to National Rail at midnight. I stayed up until midnight, then the forum froze.
Note that LNER tickets for that date have still not yet been released, pending verification of any engineering work. Avanti tickets (for the faster trains from Birmingham to Edinburgh are also still on hold).

Averting to your original idea from February to route through the Cotswolds with a break at Moreton in the Marsh that is still possible-

Southampton to Oxford at 0715- £7.05 Advance Ticket with a railcard, change and arrive M-I-M at 0918

Off Peak Day Single Oxford to Worcestershire Parkway £16.15 (allows break of journey at M-i-M)

Advance Ticket Worcestershire Parkway to Edinburgh £30.50 on any hourly direct afternoon train. Remember the trains are starting at Worcestershire Parkway so will not be arriving there busy. Being an Advance Ticket you need to choose which train you want.

So a total of £53.70 each.