The investment process after filming that checks a Dragon's Den deal before completion is called due diligence.
When a Dragon and entrepreneur agree terms on camera, the agreement is not normally the end of the transaction. Due diligence follows, allowing the investor and professional advisers to examine the company's accounts, debts, ownership, intellectual property, contracts and other relevant evidence.
This distinction explains why some deals shown on Dragon's Den do not ultimately complete, or why their terms change. The televised negotiation records an in-principle offer, while the later investigation tests whether the business matches the claims made in the pitch.
Due diligence is standard practice in investment and is not unique to the programme. It protects investors from undisclosed problems and gives both sides a clearer basis for completing a legally binding transaction.