Train questions

There is a lot to unbundle here-

Last question first- Plus Bus is a ticket within certain towns, not for inter urban travel. Generally the answer is (with railcard reductions) 1 or 2 journeys within that respective town. This is the PlusBus website to drill down further into that option- https://plusbus.info/

For inter urban travel we are still on the national £2 maximum single bus fare.

Advance Fares on all companies should now be posted. So see what you have for your proposed travel dates then look for each journey at say this Tuesday or Wednesday. Some companies close their Advance sales as little as 2 or 5 minutes before train departure. Then tomorrow (Monday) try the same exercise for that day for your planned journeys.

Yes an Advance Fare (if still available- they are capacity controlled) purchased soon before travel will almost always be at a higher rate than now- hence the above experiment.

Train Companies- no, people set way too much store on which company runs the routes. An Anytime fare will almost always be available on any operator.

Changing an Advance Ticket is generally not possible without buying a new ticket. The time on the ticket is cast in stone. If you wanted to change the time the most sensible method would be for a station to excess it up to the cheapest possible flexible fare- might be Anytime, Off Peak, Super Off Peak or (on the last 2, maybe 3, routes) an Evening Out Single for travel after 12 noon.

Getting a London to Edinburgh Advance for under £40 would need a lot of luck!

There should be an option to find flexible fares on any website or app but-
London to MIM- £46.80;
SUA to OXF- £24.30
OXF to Bath- £36.90 or Off Peak Single (OPS) after 9am £25.40, or Super Off Peak Single (SOPS) after 10am £22.70
Bath to Salisbury- £23.10
Salisbury to PTM- £24.40
PTM to Brighton- £21 or £20.10 OPS
Brighton to London (I assume you will then use either the Tube) £33.50 or £23.90 SOPS
OR
Brighton to Faringdon £30.40 (then the Elizabeth Line to LHR £13.30 more)
OR
Brighton to Gatwick (for the bus to Heathrow) £11 on Thameslink or £13.10 on Southern (also available on Thameslink)

Gatwick then bus is thus the cheapest option and as fast as routing through London.

At these Anytime prices a Britrail London Plus may well be the cheaper option.

All the above prices are quoted before railcard reductions

By the way National Express run a bus every hour from Brighton to Heathrow- 2 hours to T5, another 15 minutes to T2/3. Fares booked ahead are £12.60 or £21 on the day. So as fast as the train, and cheaper.

isn31c - thank you. This is very helpful. Looks like I have to expand my spreadsheet and do more research.