Hello, Ru4fish. Importing classes into QuickBooks Online (QBO) UK is currently unavailable. However, you can use a third-party app to help you achieve this.
I recommend exploring third-party apps that integrate seamlessly with the program to help you import classes. To get started, go to the My integrations icon at the top of your QBO dashboard and select the Find integrations option. From there, you can browse and choose an app designed to meet your specific importing needs.
I've included a screenshot for your reference:

Please feel free to revisit this thread with any updates or further questions.