@ShoebillStork What you have done is a very shady, and wrong, and you must be incredibly obtuse if you don't think so!
And the posters calling people drama queens, 'hard of thinking,' and 'pearl clutchers,' can bore off. People are entitled to views different to you and the OP you know, so just do one eh, with your sanctimonious put-downs towards people who think the OP is wrong in what she did! 
And I was on the fence until you said 'OMG he is not getting THAT much!' (A certain % of the value of the house.) But now I think you're very unreasonable, and a bit obnoxious! Why does he not deserve it? You used HIS money to add value to your house. You've got a nerve!
Some might think he deserves it seeing as how you used HIS money, without HIS permission. It's cheeky as fuck at best, and yeah, fraud and theft at worst.
If it was me, I would be honest and tell him what you did, then open an ISA for him, put £15,000 in it, and make it so he can't touch the money until he is 21. Maybe even 25. He doesn't need the money at 18. He will need it more at 21, but will be a bit more sensible with it at 25. And it will have 7 years to accrue interest.
And FFS don't steal borrow his money again! 
Hopefully, the FIL never finds out!
And I seriously can't believe there are some posters on here, who are obtuse enough to actually think there is nothing wrong with what the OP did! FFS! she didn't even ask first! She took someone else's money! 'Oh but she is putting it back now!' a few posters cry. FFS, shall we all just take money from our family members, or our employers, and keep it for TWELVE years, whilst we add value to our home, and then give it back when it suits us..... ???
Yeah right. Coz THAT won't end up with a charge for theft against us will it?! 
This fucking place sometimes. 