You'd want to set-up first your VAT in QuickBooks so you'd be able to add it in your estimates and invoices. I'd be delighted to guide you on how to do it.
Go to the Taxes menu.
Click the Setup VAT button.
From the Setup VAT page, select the beginning month of your current VAT period.
Choose your filing frequency.
Pick your VAT accounting scheme.
Enter your VAT registration number.
If your business is entitled, select the Flat Rate Scheme (FRS) checkbox.
Hit Save.
Click Ok to complete the process.
Once completed, you can now create your estimate or invoice. Here's how:
Go to the New (+) icon.
Choose either estimate or invoice.
Fill in the needed information.
Once done, click Customise and select Edit current.
Go to the Content tab and select the third section.
Mark Tax summary and click Done.
Click Print or Preview to review the invoice.
Also, I've added these articles to learn more about managing your VAT information and customizing your sales forms in QuickBooks: