This will depend on whether you have a contract with the debtor and whether they’re a business or a consumer (ie a private individual).
If a contract is in place, you can charge interest in accordance with the terms of the contract. If a contract isn’t in place, you can only charge interest on late payments if it's a business-to-business transaction and statutory interest applies. Currently, statutory interest is 8% plus the Bank of England base rate, which is subject to change. For more information, read Calculating interest on commercial debts.