If I remember correctly, there has been for years a legal provision meaning it is possible for sole traders and SME's to add a percentage to invoices that large corporations (aka clients) can't be bothered to pay promptly. Of course, when we tried, such clients reminded us that they could choose
their suppliers... Always remember my manager in the 1980's shrieking at me "we always send invoices by first class and pay bills by second class." That kind of thinking sent me self-employed forever afterwards.
I took out a county court order against a company I provided services for, I didn't add the interest because I didn't want to give them an opportunity to dispute the amount. 8 got paid because they couldn't afford to have a county court judgement against them. They eventually went bankrupt!