Travel by Train questions

Partial answers.
Canterbury and Windsor are day trips from London
Bath and London require an overnight, there is more to see and the trip from London takes longer (~2 hours).
Lundy island, no train (of course ☺). Not sure where the boat leaves from and how far that is from the nearest station.

National Rail is the one to use, it is run by the train companies. Trainline is a reseller that adds a markup.

Changing trains seems to "freak out" those who have not travelled by train before. How long it takes depends on the size of the station. Small stations, 5 minutes would be plenty. Sometimes it is literally 10 steps across a platform to the other train. Others you need to go up steps, across and down to another platform.
Big stations (like York), I prefer 10-15 minutes. In both cases if your incoming train has been a trip of 2-3 hours, or if your departing train has a long wait for the next one (>1 hour), I would add some "pad" to allow for the incoming train being late.

The earlier you buy tickets the cheaper it is. There are no hard deadlines, just when the cheaper tickets sell out.

I also read that if I book tickets 12 weeks ahead I could get up to 1/2 off the train fare.

Not quite.

Approximately 12 weeks before your date of travel, a ticket type called Advance is released. These commit you to a specific train but can be considerably cheaper than the walk-up fare (known as Anytime or Off-Peak tickets).

Advance tickets sell out and are limited in availability. Anytime and Off-Peak tickets never sell out.

Congrats on your first trip abroad!

This itinerary seems a bit problematic to me. Let's say you do London (2 day trips), go to Lundy Island, then Bath, then York then back to London.

Using rome2rio, it looks like it will take a good 4-6 hours to get to Lundy Island from either Bath or London. So you've got 2 days that are basically travel to/from Lundy Island.

Then to get from Bath to York is another longish travel day, with worst case scenario having to change trains (and stations) in London -- although the more logical train would change in Bristol. It's still at least a 4 1/2 hour ride. And that's not counting getting to and from the train stations.

Or, you could go to Bath first, then Lundy Island, then fly from Newquay to Manchester and then take a train to York and OMG it's exhausting just typing it.

Hopefully this is your FIRST trip to England, so I'd save either York or Lundy Island for your next trip and concentrate on things that are closer together. Otherwise, put this itinerary on a calendar, determine the number of days/nights in each location, and see if the travel time and logistics will work for you. Good luck!

The Two Together Card is a smaller investment, since you only pay for one card, instead of separate senior cards. Check out their line "Opt for digital and get your Railcard today." If it can be delivered as an e-ticket, then great, and a hotel address may work as a placeholder (but not for mail delivery). Otherwise, I'd buy the cards in person in a train station.

To get to Lundy Island, you have to get to Barnstaple by train. So first, you would have to reach the mainline station at Exeter and then change to the slow branch line that goes through the Devon countryside to Barnstaple. From Barnstaple, you would then need to catch a bus to either Bideford or Illfracombe - depending on where the ferry is going from to Lundy. So, being as you are going to Bath, it would be sensible to then progress west to Devon rather than return to London. https://www.landmarktrust.org.uk/lundyisland/

When you see the time given to change trains at a particular station, that has been carefully worked out depending on how big the station is. www.traveline.info is good for finding buses.

The Senior Railcard is valid at all times but the 2 Together one is not valid before 9.30am on weekdays - I think. Of course, it would cost £60 for both of you to have the Senior Railcards but only £30 for the 2 Together one - which requires photos but not the Senior one.

For short rail journeys - pay on the day. It is the longer trips that lower prices for specific trains can be had by pre- booking. Play around with www.nationalrail.co.uk with different dates v pay on the day.
https://www.seat61.com/UK-train-travel.htm

I am surprised to get all the way down here on this thread before a mention of Seat61. Along with an enormous amount of information, this blog-gone-famous discusses strategies for saving money. Privatization set loose a cascade of private train operators and some competition. It also led to a degree of chaos in schedules that is on-going. It's still a great way to travel but I would never buy a ticket without checking out Seat61.

Check out their line "Opt for digital and get your Railcard today." If it can be delivered as an e-ticket, then great,

I made that query regarding a Friends and Family Railcard and the response was that a UK based address, even for digital cards, is required to comply with financial laws. I do not believe a hotel is acceptable. The payment card used to make the purchase has to be linked to a UK residential address.