The answer is Yes and No.
Business to Business
If your customer is a business - yes you can add interest only after the invoice is overdue. You are covered for business to business invoices under the "Late Payment of Commercial Debts Act" which allows you to add 8% statutory annual interest.
You can calculate interest due to you here - https://dontpaylate.uk/
Business to Consumer
If your customer is an individual you can not add interest unless you have a clause in your original contract typically 1%-5%.
If we obtain a CCJ/Judgment against your customer, we can then add statutory interest of 8%.
Please do not hesitate to contact us today if there are any outstanding invoices you would like us to attempt to collect.