This is what I've managed to glean from searches.
As I understand it, the maximum you can hold is 50k. Above that it is not eligible to win prizes.
You have opened an account for your daughter. She / that account is eligible to win prizes. You manage her account and the NS& I will notify you of any prizes she wins. All prizes won on premium bonds are tax free.
You can cash in the money in her account by filling in an application form to transfer the funds to your account. But then, as I see it, you would be over the 50k limit and because of this, NS & I would transfer the excess to your nominated bank account.
The conveyancing solicitor will pay your deposit, fees etc and the stamp duty owed and will have to know the source of that money, for money laundering regulations purposes.
Whether the solicitor will be interested that it is over NS & I's limit for holding money in premium bonds and winning prizes I don't know but in any case NS&I would transfer the excess out of the bonds and into your nominated bank account.
You could ask the solicitor. If you were worried about potential complications with HMRC*, you could pay the house purchase deposit and take out a short-term bridging loan or use a credit card, personal loan from child's father or other family members to pay the stamp duty, leaving your daughter's bonds alone until house purchase completed. Worry about the works and the sofa afterwards.
*MSE website has this about the 50k limit - which sheds some light on the subject, but as NS&I already knows you have opened an account for your daughter, it would appear not be relevant:
What happens if I save more than £50,000?
The maximum holding limit of £50,000 is strictly enforced – if for any reason you've more invested and NS&I catch on (for example, due to different spellings of names or other missing information), it will repay any surplus back to your nominated bank account, although you may have to repay any prizes won in that time via bonds that were ineligible because of the maximum holding limit being exceeded. These prizes are then reallocated to the next eligible bond drawn in the relevant month's draw.
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