I'm glad you came here, eugeneeustace.
This error when submitting VAT return is often caused by outdated browser. We can try a few troubleshooting steps to resolve this. Let's start by using incognito browser to rule out the possibility of a web page issue. Private browsing doesn't store local files or cache.
Chrome, press Ctrl + Shift + N.
Mozilla Firefox: press CTRL + Shift + P
Safari, hold down Command + Shift + N.
If all else fails, I recommend contacting the HRMC. They can help review your user ID to make sure you can submit your tax return.
This link will route you to the HMRC website: https://www.gov.uk/contact-hmrc
Visit us again if you have other QuickBooks question. I'm always delighted to help. Have a nice day!