James,
I currently work for HSBC although I will be leaving to start training at Oxford next month. There is no limit to the amount you can borrow on a Professional Studies Loan but what you must remember is that this type of loan is unsecured. The reason why HSBC are the only Bank to offer this type of loan is that the exposure to risk is so high.
The £50,000 limit is actually based on a business agreement between HSBC Oxford Branch and OAT. In theory, the amount you can borrow is only limited to the full course fees plus £5000 for living expenses.
Providing you can prove that you need the funds for the training and have the means to pay it back, its purely at the discretion of the manager at your local branch.
If you are looking to borrow higher than £50,000 you will probably be expected to put forward a substantial sum of your own money, eg I was asked to put in just less than half for my loan of £45k. Also, any type of security will help, whether it is property or a guarantor such as parents. The less risk you impose on the bank the better your chances.
Please note that this information is purely from my experience in the past with these types of loans and is for information only.
Hope it helps.
Purmus