I am surprised this problem doesn't come up more frequently on these forums.
BBVA is a commercial bank that specializes in this type of lending in conjunction with a few of the major flight training schools.
The lending criteria is quite tight in respect of the amount of equity they will lend against. Including the proposed borrowing, the total sum shouldn't exceed 60% of the properties value. That is quite a restrictive sum for anything other than a mature mortgage on quite a valuable property.
That isn't the only criteria either. They also want to satisfy themselves that the guarantors can reasonably carry the loan payments in the event of the primary borrowers default. As a result this avenue is only going to available to those who choose it, who can also satisfy these criteria.
For borrowing these large sums of money (in the £80,000 to £90,000) range, you are either looking at savings, savings with unsecured borrowing (usually capped at £25,000,) or mortgage borrowing from another bank. In the latter case this may be a re-mortgage for the existing debt plus the required advance, or a second mortgage to cover the required loan. With the latter two, this is secured borrowing and would still have to satisfy the lenders criteria for such loans which may or may not be less restrictive than BBVA's, depending on the borrowers personal circumstances.
In summary you need to sit down with your parents and discuss the options, or go out to work and start saving.