You'd need a business case, a safety case, and a huge amount of money. That's the short answer, anyway. Business and safety cases that satisfy the ORR are a big deal and cost a lot to put together.
Strictly speaking, you don’t need a business csse for the ORR (more of that later), but you will need a business case to satisfy whoever is lending you the money (lots of it) to propose and operate the service.
What you have to demonstrate to the ORR is that your service will provide genuine benefit to passengers, and not be ‘primarily abstractive’ - ie it generates a net income for U.K. rail plc, without unduly affecting the income of existing operators. To do this you will need some complicated demand forecasting and revenue modelling, and not just say “it’s a no-brainer, innit”
You will also have to have progressed a track access application with NR, and demonstrated how the trains could fit into the timetable without affecting the contractual rights of other operators.
The safety case / licensing is another raft of documentation too. This can be ‘got round’ by contracting the whole operation out to an organisation that already has this, but then the one thing you can guaranteee out of that’s is that they will make a profit, whilst there’s no guarantee of that for you.
