Hi Danielle and welcome to the Disney Parks Moms Panel.
It's always lovely to be able to answer a question from a fellow Brit.
You can absolutely use cash to pay for the purchases made on your
MagicBands (before they are charged to the credit card you have put down, when you check-out).
Simply stop by the Front Desk at your Disney Resort hotel and ask for a summary of the purchases which have been charged to your room. You can then clear the balance and start again. You can do this daily if you prefer or every few days, if it's more convenient.
I often do this using either cash or Disney Gift Cards so that I can keep an eye on how much we are spending.
If you don't want anything at all to be charged to your credit card, be sure to stop by the Front Desk the evening before you will be checking out. During previous visits to the Walt Disney World Resort, I have normally found that a credit card receipt has been posted under my hotel room door when I wake up on the day I am checking out detailing the charges made to my credit card. Don't leave it until the day of departure as it may be too late.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma