Let me address the latest comment by the OP.
Darrellandkimberly wrote:
"The trips we are planning are Salisbury/Stonehenge, bath, Stratford upon avon, Canterbury, cambridge, york/whitby (this will be a 3 day trip and planning on renting a car in york and driving to whitby. ), Hever castle, maybe Windsor castle
We also talked about renting a car for each of the day trips but I am thinking that parking will be expensive and make it not worth it."
I will suggest that you do your trips in this order.
Train from London to Bath.
Stay 2 nights.
Train or bus for the short hop to Salisbury.
Stay 2 nights.
Train to Stratford-Upon-Avon.
Stay 2 nights.
Train back down to Oxford--just as interesting as Cambridge.
Stay 1 or 2 nights.
Train or bus back to London. The bus is really cheap--can buy tickets on the day of travel, just walk up to the window and buy.
The schedule above will all be doable by train. It is my preferred way to travel---That doesn't mean there aren't plenty of people on this forum who love to rent cars in England.
Now----before you say you don't need a hotel room at these places, that you already have a place to stay in London and want to do these as day trips........
I will say these are really cool towns that you will enjoy a lot more if you at least stay a night or two.
Each town is good for walking around and enjoying, visiting a pub.
I checked your other threads to see how long you will be in London.
https://community.ricksteves.com/travel-forum/england/how-much-money-to-put-on-oyster-card
"We will be spending 3 to 4 weeks based in London with around 6 or 7 day trips and one 3 day trip to Whitby before going to Paris for 3 or 4 days. We are staying in London with a family member who has extra oyster cards."
The last sentence explains your desire to do these as day trips.
However...cheap hotels can be found in these locations, B&B's, or hostels.
So it would not be that big of a hit to your budget.
Let me know if you want a website for hostels.