Andrea,
a difficulty using the Family and Friends Railcard in your situation is that you are all adults. There is a restriction, "Railcard holder must travel with a child for discount to apply" so the tickets you bought with that Railcard would be invalid. Unless you could get a parent to part with their child for the time you will be on trains, sort of rent-a-kid....
As a group of 4, the Groupsave-4 tickets would help on walk-up fares, but all of your journeys are long enough you will want, I expect, to save the most money by carefully planning each leg and buying what are called "Advance" tickets well in advance, knowing that Advance tickets are only valid on the exact train at the exact time purchased, and only good as a bookmark if you don't make that train for whatever reason.
You could make those cheaper tickets even cheaper with a pair of Two Together railcards. They will get you 1/3 off almost all tickets on trains (not preserved railways or private trains or a very few other exceptions) as long as you meet a few restrictions - you need to provide photos, you must travel together on each leg (the Railcard is not valid if only one of you is on the train), you can't start a journey earlier than 9:30 M-F (any time on a weekend or Bank Holiday), and for the £30 per card to pay off you need to totally spend at least £100 per couple on tickets. All the details at https://www.twotogether-railcard.co.uk/
You might be thinking, based on your question, about the Senior Railcard. In your case I don't think it is worth it UNLESS you must start before 9:30 on your trips. It costs £30 per person (not per couple like above), you don't need a photo but do need to prove your age(s) and one of your group doesn't qualify. You can split up (I know you and you husband but not the other couple), so you really need to consider your choices.
You can't get a train to Woodstock, if the Woodstock you mean is the one next to Blenheim Palace. Oxford is the nearest main station, and buses and taxis to Blenheim are plentiful there. If you arrive at Blenheim by bus you even get a discount.