I want to import a backup file from QuickBooks Desktop Pro to Quickbooks online. Has anybody (outside the UK) succeeded in doing this?

Hello, @m-lyons. 

The option to import your data from QuickBooks Desktop (QBDT) to QuickBooks Online International is unavailable in QuickBooks Online. 

You'll need to manually export the data from QBDT, and import it in QBO. You can use either CSV (Comma Separated Values) or XLS/XLSX to import information to QBO. The following are the information you can export:

• Customers
• Vendors/Suppliers
• Products and services
• Invoices
• Bills

Check this article for more information: Common questions about importing data to QuickBooks Online.

However, you may also check the following article if you’d like to use a third-party app in converting your data: Import Data Conversion to QuickBooks Online - Frequently Asked Questions. 

Let me know if there’s anything that you need. I’m always here to help. Stay safe!

@m-lyons 

Which QB version do you have? UK, US, CA, or else?

Where is your company located?

You may refer to the following link with similar case

https://quickbooks.intuit.com/learn-support/global/importing-and-exporting-data/how-to-i-transfer-from-qb-desktop-pro-2012-to-qb-online/01/594888#M156