I have more than one business so I take a different approach depending on whether or not its a service or actual goods being sold.
If sending physical goods, I always expect payment before dispatch unless its a reputable business and they ask for 30 days in return for an official Purchase Order. Either way, for the first order I send a pro-forma for payment up front. For future orders, I might give them 30 days.
For services - design or anything bespoke - 50% up front, 50% on completion seems the norm. If its a reputable business, then 30 days is acceptable as long as they supply a Purchase Order or as a minimum an email agreeing to the costs, terms etc.
For late payers - remind them twice, then either shut down their services, or deactivate (as applicable) etc.
For persistent non-payers -
1. Final notice with a warning that you will charge 8.5% interest and £70 collection fee if not paid within 14 days as per the Late Payment of Commercial Debts (interest) Act 1998.
See this page:
https://payontime.co.uk/late-payment-legislation-interest-calculators
2. If you still don't see the colour of their money then next stop is:
http://www.moneyclaim.gov.uk
Make sure you have signed terms / contract or at least have emails showing they placed the order and agreed to pay the price etc as you will need these as evidence for the Small Claims Court.