Glad to have you here in the Community, @bracknbarn.
Let's perform some troubleshooting steps so you can submit your VAT return successfully.
There are times that the browser is full of frequently-accessed page resources, thus causing websites, like QuickBooks Online (QBO), to act weirdly. Let's try logging into your QBO account using a private browser.
To use a private browser, here's how:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once logged in, go back and try to submit your VAT return again to double-check. You can check out this article for more guidance: Submit a VAT Return in QuickBooks.