How do I send an invoice with the total balance on but only ask for 50% deposit with the second half payable once the job is complete?

Great to have you here, @kookaburrashutte.

To send an invoice with the total balance displayed but only ask for a 50% deposit in QuickBooks Online UK, you can follow these steps:

1. Log in to your QuickBooks Online UK account.
2. Click on the "+ New" button or the "Create" menu.
3. Select "Invoice" under the "Customers" section.
4. Fill in the relevant customer information.
5. Add the products or services you are invoicing for with their quantities and prices.
6. In the "Payment terms" section, select "Custom" from the dropdown menu.
7. Enter the payment terms description, such as "50% deposit due now, balance due upon completion."
8. Add a line item to the invoice that represents the 50% deposit. You can create a custom product or service item specifically for the deposit.
9. Calculate the total balance amount (100% of the job cost) and include it on the invoice, even though you are only asking for a 50% deposit.
10. Add a note to the invoice's message or memo section explaining the payment terms and that the balance will be due upon completion.
11. Save and Send the Invoice.

Once done, you can use this customer profile to record and receive invoice payments based on their own currency. 

I’m also attaching these articles for your overview and how Multicurrency works. 

Please know that I'm just a post away should you have any other questions concerning QuickBooks. I'd be pleased to help you out. Take care!