oyster card

I believe it is only valid in London. I usually put 35 pounds on mine for a five day stay. It's very easy to top up your card should you need to do so.

Just use your debit cards or phones to tap in and out on the tube and trains, and in only on the buses.

if I use my debit card I will be charged international fees which I am trying to avoid as many as possible.

The Zone 1 and 2 fare cap (ie- Central London) is £8.50 a day.

So 4 days in London if you reach the cap is £34 + your £5.60 in on arrival day from LHR= £40.

Prices go up in March by 4.8% so as your trip is in August about £43 is in theory the most you can spend if you don't go to places in the outer zones like Hampton Court or Greenwich.

The Oyster Card is only good for London. If using buses anywhere else in the country then any bus accepts cash.

Some bus companies also have capping in place- for instance East Dean/ Seven Sisters if you use the bus chances are it will be on Brighton and Hove buses. On B and H when using contactless you swipe on AND off the bus when using contactless to use their cap (if less than the cap you will be charged for however many £3 fares you use).
I believe First Bus in the Bath area also have Tap and Cap so that will also be swipe on and off (but check that).

Stagecoach buses for instance across the Country (and in the Eastbourne area there are some Stagecoach routes) don't have tap and cap so you do in London- tap on only. So it varies operator by operator. If in a tap and cap area then I pay by cash to avoid the computer forgetting to apply a cap and thus over-charging me.

You can pay for rail tickets at any manned booking office in cash, but most machines don't accept cash. But I assume most if not all your rail tickets will have been purchased beforehand at home.

Likewise any taxi you take should accept cash- they may not necessarily like it but under taxi licensing rules they are required to.

The standard answer on this forum is that the international card fees are so small that they are total pocket money compared to the overall cost of the vacation.

Oyster card is only good in London.
Put 25 to 30 pounds on it.
As you use up some of that, more can be added to the card toward the middle of the week.
It's easy to do at one of the machines that dispense the Oyster card.

Good information about the Oyster card here on this recent thread:
https://community.ricksteves.com/travel-forum/england/oyster-card-purchase-amount

I use the Oyster card on transportation because getting it out all the time I'd rather lose an Oyster card than my credit or debit card.
I don't carry a phone.

Oyster is used on the tube--tap in & tap out.
The London buses--tap in (on the pad near the driver) but don't tap out when you get off the bus.
The water taxi/Uber Thames Clipper--tap in, tap out.

Oyster cards cost you £7 upfront before you do any travel. For a 4 day visit this may well be under the fees you would pay by using a card directly.

I didn't use my debit card for the Tube last time I was in London but I did use a credit card (American Express) to tap in/out. There were no transaction fees associated with the tap in/tap out charges. I do usually have a transaction fee on regular charges. You might check with your CC company to see if anyone can give you an answer. You could also use it once to see if a charge appeared then decide on whether to continue or go with the Oyster.

TBH, the tap in/out was so much easier than the Oyster just for the reason you are saying....having to figure out how much to put on the card! I did start out last visit using up the remainder of what was on my Oyster from 2018.

Fees would depend upon your card.

I would caution against using a debit card that charges fees, just due to the variety of fees, which then comes down to the specifics of your bank. Some cards charge a flat use fee in addition to an international transaction fee, usually a percentage of the charge. If you understand the charges and are comfortable with them, fine; keep in mind, you are not making a charge each time you tap, but only once per day after the travel day is done.

Using a credit card or phone/watch is better. Even with a foreign transaction fee (FTF), typically 3%, it is cheaper to use the card as opposed buying an Oyster Card at 7 GBP, it would take you about a Month to recoup that fee paying a FTF.

The other advantage of tap to pay over Oyster is there is no question of what to load, and no hassle to get excess funds refunded.