You dont have to give 5% of that growth to HSBC the 2nd way
Borrowing on the government balance sheet isn't free, there are costs for the Debt Management Office and the Gilt Edged Market Makers.
What matters is the overall costs of different ways of financing the asset, and different risks.
Even if leasing has a higher estimated cost than borrowing, that might be regarded as a good deal if it reduces financial risk.
Look at it also from the perspective of the investors, which are likely to be the same organisations irrespective of the method of financing. They will have different risk profiles for investment in a ROSCO, where the risk is confined to the trains and their financing, and investment in gilts, where they are exposed to future trends in government financing as a whole. And they will have read the OBR's financial risks and sustainability report!
None of this is done with certainty: it all involves assumptions on future movements, over many years, in economic growth, inflation and interest rates. It can't be boiled down to fees to financial intermediaries which is just one part of the cost.