Statutory credits are the "lender of last resort" for biodiversity net gain. In other words, they are how you achieve net gain if all other routes have been exhausted. They should be the last thing you consider, as part of the mitigation hierarchy. However, if you have avoided what harm you can, minimised what harm you cannot avoid, compensated on-site where possible, and have been unable to find off-site units, they are there to ensure you can proceed with your development.

You have to buy two statutory credits to offset one biodiversity unit.

Where you think you need to use credits, you can purchase them from the government. You should not do this until you are certain that your plan to use credits will be accepted as the government will not refund credit purchases. You can read more about statutory credits on the government's website.