Good day, dougpeel!

Adding a VAT number to your invoices in QuickBooks Online is pretty easy. 

You'll need to enter your VAT registration number first once you set up your VAT. This should, thereafter, populate automatically on the invoices. To setup your VAT, please follow these steps:

1. From the left navigation bar, choose Taxes.
2. Click the Edit VAT drop down then click Edit Settings.
3. Make necessary changes then enter the VAT registration number.
4. Click Save.

If you'll set up your VAT for the first time, please see this link for the detailed instructions: Set up and edit VAT settings, VAT codes and VAT rates (UK only)

You can also visit this link for more information about getting started with VAT: VAT getting started

With regard to your invoices, you can edit which details appear on it. Let me show you how: 

1. Go to the Gear icon.
2. Under the Your Company heading , choose Custom Form Styles.
3. You will then see a standard template which is for your invoices.
4. Select the blue hyperlink that shows as Edit. 
5. Click on the Content tab and click on the bottom half of your invoice on the right hand side f the screen. Here, it will allow you to add additional details int o the box and a footer.
6. Click Done.

Here's a link for more information about Customising your invoices: Why Should I Customize My Invoices?

I've also attached screenshots for your reference.

Keep me posted if you have any other questions or clarifications. I'm ready to help. Have a  nice day!