Essentially you are applying for a bank loan and if you do have a poor credit rating you do run the risk of being refused just like anyone else in the same position.
However, HSBC do know about the TRSS scheme and it is their money they are lending to you in the knowledge that you are guaranteed a job with easyJet on sucessful completion of the rating.
I'm no guru when it comes to how bank's work, but it could work in your favour despite a poor credit rating as you are a fairly safe bet for them having gone through the selection.