I am from the UK but am apparently using the USA version of Adobe Stock. How do I change this??

you can start with the following info to see if it helps, but just in case, i'm moving your message to the stock forum because there might be something specific about stock.

The country associated with your adobe id must match the store you purchase from and must match the payment address. otherwise, change what's necessary:

change your country on adobe.com, https://www.adobe.com/international/selector/

change country associated with adobe id - https://helpx.adobe.com/x-productkb/policy-pricing/change-country-associated-with-adobe-id.html

change credit card and/or billing address, https://helpx.adobe.com/manage-account/using/update-billing-information.html

change currency - https://forums.adobe.com/thread/1999694

change store - http://www.adobe.com/store_selector/storeselector.html

change adobe.com website language - https://forums.adobe.com/thread/1999693

change age - https://helpx.adobe.com/nz/creative-cloud/kb/you-eligible-cc.html

<moved from account, payment & plan>