switch on multicurrency

QBSE UK version only supports the single currency (i.e GBP). Consider having a Wise business account to deal with multi currency transactions as a workaround.

https://wise.prf.hn/l/64dl92k

Otherwise, you should switching to QBO Essential or Plus to enable the multi currency features.

https://quickbooks.grsm.io/UK

Thanks for checking in with us, Theboxofcheese. 

The option to turn on Multicurrency in QuickBooks Self-Employed (QBSE) is unavailable. This is the reason why encounter the error message when sending the payment. As a workaround, you'll have to manually convert those amounts from your home currency to GBP. See the Categorize and edit transactions in QuickBooks Self-Employed article on how to review your transactions in QBSE. 

You may consider using QuickBooks Online to be able to use the multi-currency feature. This will track your foreign currency transactions including any exchange gains or losses. To know more about this, you can refer to the About Multicurrency article. Once you already downloaded all the data you need, you can proceed with signing up for QuickBooks Online. I recommend canceling QBSE after transferring your data over.

For more information, as well as how to start moving your transactions to QBO, I recommend the following article: Switch from QuickBooks Self-Employed to QuickBooks Online.

Drop me a comment below if you have any other questions about the migration process. I'll be happy to help you some more. You have a good one. 

Thanks.  I want to use GBP as my only currency but unwittingly set up with USD.  I can't work out how to change my default from USD to GBP.  Any ideas?

I am pretty certain I have got the UK version

Navigate to your Billing & Subscription menu. How much is your subscription bill?

I can't do anything from my billing page

apart from cancel.  Very tempted.

Share the screenshot of your Billing & Subscription menu to make sure.