A bank in the UK will nearly always apply a maximum lending ceiling of £25,000 for unsecured borrowing. Even then it will want to be convinced that you are both a good credit risk (from your established credit history report) and able to satisfy the repayment schedule from your income. They are not really interested in what you might be earning as a result of the loan, nor within certain bounds do they much care what you want the money for, other than certain regulatory restrictions that apply to specific types of borrowing.
For sums larger than this, you are almost certainly going to be looking at secured borrowing. The "security" is normally an asset acceptable to the bank. Nearly always this involves a house where the unencumbered portion of the value (the equity) exceeds the value of the loan being sought. If agreement is reached, the lender (bank) will take a legal charge over the property. In essence this becomes a second mortgage, and allows the lender to apply for a forced sale of the property in the event the borrower defaults on the terms of the loan.
Notwithstanding this security, the lender will also want to satisfy themselves that the borrower can still meet the loan payments from their income.
Where a borrower doesn't have the necessary security or income or both, a guarantor can in some circumstances provide the necessary surity. Provided the guarantor can satisfy the lenders normal borrowing criteria (with regards to security and income,) then they agree that in the event of the primary borrower defaulting on the loan, they accept the same liability wholly as if they were the primary borrower.
There is no particular upper limit for this type of borrowing save as to the income multiples of either or the borrower and guarantor, or the percentage of equity in the property or securing asset.
In the UK and Spain, a bank called BBVA specializes in this type of secured lending and has tailored products that it markets through the main training providers. They may lend the entire course fees, and if required, monies to cover living costs during the course. They will also provide a "repayment holiday" of around 18-24 months to enable no repayments to be required for the duration of the course and a short period thereafter. They might if required, also allow a further repayment holiday of 25% of the normal repayment instalments for the first 2 years of the repayment schedule. There are additional set up costs (fees) for the loan. You should also understand that the "repayment holiday" is simply a vehicle to allow a staged increase in the repayment schedule. The variable rate interest (currently 3% set at Base rate +2.5%) will continue to accrue from the date the monies are drawn down. For any given repayment term that clearly results in a much higher level of instalments for the rest of the repayment term beyond the "holiday"!
Just as with any secured lending the rate of interest can rise substantially if the Bank of England base rate rises (the margin currently +2.5%) will remain the same margin throughout the loan. The bank will want to ensure that any borrowing plus existing mortage (there can only be one) does not exceed around 60% of the current market value of the property. They will also want to ensure the lender, or more likely the guarantor, can afford the normal schedule of repayments in the event that the primary borrower defaults.
This type of borrowing is the most common vehicle with the big integrated providers, and clearly it comes with significant risk. It won't suit everybody, and in many cases most would be borrowers wouldn't qualify for it.
For the British airways FPP scheme, the airline stated that it might in certain circumstances stand as guarantor for qualified applicants who couldn't raise the necessary security through their own means.
Secured borrowing is (subject to their own terms,) potentially available to qualified, quality, and suitable applicants, though most major lending institutions, although the one mentioned specializes a product in this particular arena. The borrowing criteria are going to be broadly similar.