Under the Late Payment of Commercial Debts (Interest) Act 1998, a business can charge simple interest at 8% above the Bank of England "reference rate" on late payments from another business or a public body. The reference rate is base rate on 31 December (for debts that become late January–June) or 30 June (July–December), and it is fixed for the life of the debt.
Interest runs from the day after the due date until the day the invoice is paid. It is automatic for B2B contracts — you do not need a clause — unless the contract sets its own interest rate that counts as a "substantial remedy". It does not apply to sales to consumers.
Example
A £2,000 invoice at 11.75% accrues £0.64 of interest a day; after 30 days that is £19.32.