If your UK provider doesn’t have a reasonable add-on, as said, does your phone take dual esim? The esim from e.g. Airalo are data only (i.e. Whatsapp calling) and with dual the uk sim will take phone calls and SMS so need to turn roaming off if its expensive.
If you aren’t tied to a contract and don’t need to keep your number, Lebara do cheap payg with reasonable add-ons … 15 days, 5 GB data and 100 minutes of calls is £10.
“We are heading to Atlanta and Tennessee, assuming the damage from the recent hurricane has not affected the trip too much”
Yeah very genuinely you need to check that carefully. Anywhere in the Appalachian mountains north of Atlanta are still in major disaster recovery mode, major roads closed, minor roads destroyed, airlifting supplies. If it’s a city trip, that’s fine, but I would rearrange anything that looks like it might have been affected.