Hello Carolyn and thanks for asking the Disney Parks Moms Panel such an important question.
All Guests visiting Walt Disney World Resort must strictly adhere to Florida Law which currently states that individuals must be 21 years old or over to purchase alcohol and sometimes even the most eligible looking adult gets asks to proof of age; yes that has happened even to me!
As a fellow Brit I do understand your concerns about not wishing to carry a passport plus a drivers licence at all times; nobody wants that kind of worry when they're enjoying a magical holiday!
So I am very happy to share this
International Travel ~ Frequently Asked Questions link which clearly says you can indeed use a valid photo ID such as a UK drivers licence
combined with either a paper copy of your passport or a screen shot of it saved onto a mobile phone; currently a foreign (non US) drivers licence on its own
isn't accepted as proof of ID.
If I can help sprinkle any extra pixie dust onto an upcoming holiday to Walt Disney World Resort please pop back to ask me another question.
Have a great Disney day & chEARs to you!
~ Jane