I'm probably missing something here (which is good since I'm well into the course now!) or just being naive, but I don't see how this is a problem. Say, for example, you start working for Easyjet; you're earning a decent living wage and the bond payments are being accounted for on top of that. The banks can see you're earning and you've got the means to pay off the bond as planned, so why wouldn't they talk to you? You've got a decent job, your big training debt is under control and you're a potential customer - I would have liked to think they'd be happy. Of course, if you lost your job you'd be up the proverbial creek without a paddle, but I've never heard of anyone that's gone through and then done that.
You say you asked HSBC these questions, you've been told that the implication will be nobody will want to give you a mortgage but "HSBC have arranged mortgages for ex-cadets". The cynic in me says "how convenient!" They would say that wouldn't they...
EDIT: I do see the point, though, that the arrangement with HSBC is unique in that nobody else gets a loan that size unsecured, which might scare off potential mortgage lenders who know nothing about it. HSBC do know about it and the associated risks, of course, and they're much happier to sort out mortgages knowing that debt exists on that basis. Hmmmmm. Looks like I'm going to be owned by HSBC for much longer than I thought, then!