us iCloud account in the UK

Hello ian816,

That's a wonderful question! When using Family Sharing, the country/region associated to the Apple ID's would have to be set to the same region. There isn't a way to temporarily set a family members region to a different country. If the family member needs to change their Apple ID country or region, that can be done from an iOS or iPadOS device using the steps listed below. These steps, along with the steps for changing the country or region from other devices can also be fund in the following article.

Use your iPhone, iPad, or iPod touch to change your region

1. "Open the Settings app. 
2. Tap your name, then tap Media & Purchases.
3. Tap View Account. You might be asked to sign in.
4. Tap Country/Region.
5. Tap Change Country or Region.
6. Tap your new country or region, then review the Terms & Conditions.
7. Tap Agree in the upper-right corner, then tap Agree again to confirm.
8. Select a payment method and enter your new payment information and billing address, then tap Next. You must enter a valid payment method for your new country or region.* Learn more about how to change or remove your payment information."

Change your Apple ID country or region - Apple Support

Take care!