Forget this £25k figure.
First year salary around £34k (composed of salary+flight pay). Security bond of 12k also paid back. This takes care of the principal of the loan (84k). Accrued interest for your loan will also be paid back, but this ultimately comes out of your salary (the 34k), and whilst not insignificant, is incomparable to the repayments that would be required of other programmes (CTC Wings for example), where your security bond is not repaid. Ie. repayment costs in excess of 84k are the cost of credit of the loan. Payments are made to a predesignated schedule and overpayments are allowed.
As alluded to by a few others, to achieve the same take home pay as the FPP acheives were your bond not repaid, you would need to earn a fair bit more. I'm not going to calculate it, but it'd be in excess of £50k.