I'll get you pointed in the right direction so you can notify HMRC for your PAYE (Pay As You Earn) tax.
QuickBooks Online allows you to turn on the RTI reporting feature to notify HMRC if you've run payroll. However, since you no longer have employees, you'll need to reach out to HMRC directly. This way, they'll be informed regarding the current status of your business.
In addition, here are some resources that you can scan through to learn more about submitting RTI and FPS to HMRC in QuickBooks Online: