Rail pass vs train tickets

You'll be "based in London," so does that mean you want to take three separate trips from London to each destination and then back to London? Will these be same-day trips or will some be overnight? (I'd recommend at least an overnight in York if you can.)

The National Rail website is the best place to start for timetables and fares. Look up the specific journeys and dates you want, and the fare options. Lower-cost fares will lock you into a specific train and time, so be careful. May schedules and fares might not be available yet, so wait another month or so if you need to. "Day return" fares (meaning you come back on the same day) are almost the same price as "single" (one-way) fares, but an overnight stay will require a "return" (roundtrip) fare. Fares for travel before and after rush hours are cheaper.

When you've figured out the specific dates and times you want, add up the fares and compare to the cost of the rail pass you're considering. Chances are it will cost less to pay for each trip individually. The National Rail website will take you to whatever rail line is running each train and selling the tickets, so you can buy ahead of time.

The Man in Seat 61 has a great deal of information about rail travel.

Windsor is on the edge of London, do that as a a day trip.
York is two hours by fast train (125 mph), this deserves at least one night.
The Isle of Wight is an unusual choice. Train to Portsmouth and then ferry. What do you want to see on the Isle of Wight? How do you plan to get around. It has one rail line, otherwise buses or a hire car.

"The rail system, to me, is very complicated and confusing." - can you tell us what you find complicated and confusing? If you want to get from A to B, you go to the National Rail website ( http://www.nationalrail.co.uk/ ) and look up trains from A to B. This will give you times and prices, and you can see if it is practical. It helps if you know where the station are in A and B and if they are near where you want to visit. You can see this on a map.

Timetables change twice a year, in mid May and in mid December, always on a Sunday.

This year I expect the change on either the 12th or, more likely, the 19th. I would know but now that I am retired I don't keep up with it.

There may be a fair number of changes this May because many of the December changes were deferred due to schedule debacle last May when the government royally screwed up trains and people suffered for months.

This year it should be much smoother, but there may be changes beyond what you can find at the moment.

Nothing is released beyond the 6th of April at the moment. Give it a bit of time and it will be there eventually.

Look in March and you'll get a good idea of Advance pricing and routing and times and duration.

The summary above explains it well.

You will likely use Kings Cross to York on LNER.
Waterloo to Portsmouth for the ferry to the Isle of Wight on South West Rail (watch out for strikes).
Depending on where you are starting from either Waterloo to Windsor and Eton Riverside (much slower) on South West Rail (watch out for strikes) or Paddington via Slough to Windsor and Eton Central on GWR (much faster and cheaper).

This year I expect the change on either the 12th or, more likely, the 19th. I would know but now that I am retired I don't keep up with it.

A couple of printed timetables I have say they are valid to 18 May, so looks like the change is on 19 May.

Just to emphasize: Advance tickets (with a capital A) are not just any tickets bought in advance. They are heavily discounted fares that go on sale about 11 weeks before travel (so, they're not on sale yet for your travel dates). You must take the EXACT train you booked, but the fares are VERY low. If you can accept the restrictions, Advance fares will beat the price of a pass by a long shot.

As said above, there are also various other kinds of discounts, like day returns. However, if you want full flexibility, a rail pass will probably be cheaper than last minute tickets bought before travel.

Start with the National Rail website: http://www.nationalrail.co.uk/. Look at your routes for tomorrow (be careful to check the date and time, so you're not inadvertently searching for trains in the middle of the night). This will give you an idea of the last minute prices. Then, look about 10-11 weeks ahead, to see the Advance fares, and the various other fares and their restrictions. Compare all this with the price of a rail pass. That's the only way to know which is cheaper and easier for your needs.

If you do want to buy individual tickets, follow the link in the National Rail website to take you directly to the ticketing website. Some tickets can be printed at home; some can be put on your phone via an app; and some will need to be picked up at a station. The instructions are clear for each ticket; just be sure you understand what's required for each one.

Our host, Rick, has a lot of free pages of travel advice. He is a particular expert on rail travel (although his agency sells rail passes!) Start here, https://www.ricksteves.com/travel-tips/transportation/trains
or use the blue menu top left.