Some lenders want to see the accrual of funds, however, most will leave it to the conveyancer to confirm source of funds.This.
Gift it to your daughter in advance and then it becomes savings, for her to do as she wishes. No need to tell the lender if she chooses to use it for the purchase of the house.
I’d be wary of hiding the gift on a mortgage application as it’s fraud which is, of course, a criminal offence. The gifted deposit letter really is no big deal in comparison.