You could have asked for a meeting with the 'someone higher'. At least the three of you could have discussed it there and then.
I wouldn't think it unreasonable for someone to ask for security, and I presume they mean a house, for that sort of money.
Two other options:
Try your luck at a HSBC branch where you know they have accepted these for pilots. Obviously, you'll have to travel to somewhere like Oxford. Making appointments and chatting on the phone prior to your visit may indicate if it will be worthwhile. Also ask about security and loan amounts prior to the trip.
Go back to the HSBC branch and ask for details on the amount that you can get unsecured.
Hope that helps.