We haven't got any reports about the missing Submit button when sending VAT returns in QBO.
Let's try some browser troubleshooting steps to check if this is caused by a web page issue. To do that, open a private or an alternate browser. Check out these keyboard shortcuts:
Google Chrome: press Ctrl+Shift+N
Mozilla Firefox: press Ctrl+Shift+P
Internet Explorer: press Ctrl+Shift+P
Safari: press Command+Shift+N
Large data in the cache can potentially cause performance issues in QuickBooks Online. Clear your browser's cache to force the system to refresh, removing any roadblocks that are creating the problem.