I was approved by HSBC for a professional studies loan of £25.5k UNsecured.
1 Go prepared as others have said. Set out exactly what your costs will be. Know where you will study and by which route.
2 Again as others have said, sell yourself, this IS an interview.
3 One which has not been suggested. Choose an HSBC close to the FTO of your choice, they will, I am sure, be VERY used to your proposal, as a large proportion of the students at their local FTO will have approached them. They will know the score well, and your potential as far as future business with them is concerned. They are in the business of making money out of you, make no mistake, but they know that if they make you happy, they are likely to keep you as once you have that big jet job, they'll be making plenty of money from you.
Also to compare CDLs and HSBC prof studies loans, do your sums on the costs of borrowing too. Although the CDL offers you an interest and repayment holiday, the rates after that are considerably higher than the base rate +2% added to the PSL (making 6% at the mo..) PSL also offers a repayment holiday until 6 months after the end of syudy, though not interest free.
Good luck....