Let me get the help you need in processing payroll in QuickBooks Online (QBO). To get started, I want to ensure we're on the same page. Thus, I'd like to verify what payroll service you currently subscribe, since this matters. Also, have you received an error upon editing the Pay date?
When you're using QuickBooks Online Advanced Payroll, you have the option to edit the Pay date. However, if it's under QuickBooks Online Standard Payroll, you can't.
In the meantime, while waiting for the information I've asked above, I recommend checking these articles to know more about the version of payroll in QBO UK: