We will need to get from Bath to London at the conclusion of our Rick Steves tour. We would also like to take a day-trip to Windsor (from London), and then get from a London hotel to Heathrow. Is it necessary to make train arrangements now (2 months before needed) or can it be done when in England?
In short all these trains can be booked on the day up to time of departure. Windsor there is no advantage to booking ahead. Bath to London will be cheaper booking ahead, but there are always good value advance fares up to the previous night. Dearer than booking 2 months ahead, but still good value. So if it is a week long tour book the day you arrive in Bath if you wish.
Just depends how cheap a fare you want to secure.
IF you’re taking the Heathrow Express train from Paddington Station to Heathrow for your flight home (you’ll need to get to Paddington), the earlier you book your tickets, the cheaper they are. They sell a limited amount of super cheap £5.50 tickets 2 or 3 months ahead of time, but even if it’s too late now for your date, August still has some one-way tickets for a discounted £16.50 or £20.00, depending on date. It’s not a savings of millions of dollars, but it’s still a savings, if you know your date. The ticket is valid that whole day, so you’re not committed to an exact time. https://www.heathrowexpress.com/?CMP=PS-US-HEX002&tid=WEXT13298&CMP=PSB200&utm_source=google&utm_medium=PPC&utm_campaign=PPCWMGoogle&gad=1&gclid=EAIaIQobChMIn8Gmtb_G_wIVzBmtBh0usQRwEAAYASAAEgI1efD_BwE&gclsrc=aw.ds#/
Don't automatically assume the Heathrow Express is the best way to get to Heathrow. It really depends on where you are staying. There are many other ways to get to Heathrow and none have to be bought in advance.
Tickets to Windsor is considered a commuter train so the price is the same.
The only ticket you could buy in advance, the Bath to London route, would save same buying early but more than likely you could get it once in the UK. (If you plan to travel the last weekend in August, be aware it is a holiday weekend and the trains could be very busy.)